Well, hopefully something here helps clear some things up on all this for you. It definitely can be very confusing, especially with so many different products & ideas out there. The basics (from what we found during testing & development + customer feedback) is this:
SSM style bars or any bars with a very small nylon or delrin or especially aluminum sleeve will not only tear up bolt holes and welds on the brackets, but they bind up the suspension a LOT.
The brackets that lower the rear mounting point will change the INSTANT CENTER. That is the imaginary meeting point if you draw lines thru the mounting points of the uppers & lowers (front to back on the car) as you would look at it from the side. They were designed to get extra front end lift on slower cars, in the 12-14 sec range.
That was the "good" point of them. The "bad" points are that they can easily come loose if just bolted on. I believe NHRA/IHRA may have put in the rules they need to be welded on, after seeing too many break off or come apart & make a BIG mess on the starting line (if lucky not to happen down track). If you weld them on, they must be EXACTLY the same side to side. Anyone can EASILY get them out of whack from each other because the rearend can be moved just about any position as you try to line them up. If this is off, the car will drive to one side, handle poorly, and could cause a wreck. Even if they are the same side to side, depending on how they are rotated/located on the housing, they can ALSO effect the pinion angle. With both lower rear mounting points loose like that (before welding), you can get almost any pinion angle setting you want (or don't want). I have heard some cars having so much pinion angle put into it (from rotating the brackets back too far) it needed a longer driveshaft to reach the transmission! This can get VERY tricky for people unfamiliar with putting these on.
On faster cars, the drop down brackets will "hit" the tires too hard & usually spin them and/or react violently. They can also make the car move around a lot more under braking at the top end. If the suspension is working properly, there is really not much benefit at all with these, especially considering all the downsides associated with them. We personally don't recommend these to anyone, except in some cases where the car has been lowered a lot & need to fix a terrible instant center situation.
Adjustable uppers will never have any noticeable effect on instant center, and will just change pinion angle. They can also help center up the rearend in the car, and in "old" days were used a lot to preload the chassis.
Some after-market arms have such a small poly bushing, they smash or split when under power very easily. There is not enough cross-section there to handle the extreme force put on them. Then the car is all over the track, and becomes very dangerous & hard to control. Only way to tell that is the issue is to take them off the car & inspect them.
We use a bigger & tougher poly that has graphite in it & a custom fit that won't bind or squeak like the cheaper mass-produced "poly" upgrade bushings can. And for the faster cars that still want to drive on the street AND go really fast, we are almost finished with an updated design of our delrin bushing upgrade we had for years. This will allow more articulation (like a rod end) for even better suspension movement than our poly bushings we currently use. This will also provide an almost solid connection front to back like a true 4-link for instant power transfer, but gives just enough to not tear brackets off or beat you out of the car like rod ends would/could do. Look for updates on TB.com and/or our site coming soon!
On the rod end style, that is more for "race-only" setups. They will ride a lot rougher on the street, and every little bump is transmitted immediately thru to the chassis. That also can oval out bolt holes and rip brackets off the frame. Anyone running this setup on the track needs to check welds on brackets & brace it all up as much as possible. These are things you do NOT want breaking or coming loose. When you start going 125 mph +, you might want to think STRENGTH more than price or weight savings.
On round tube vs rectangle tube, there are several differences. We use rectangle tube on our arms because we get a MUCH stronger weld attaching the ends. The reason is the actual amount of weld or surface contact you get on round vs tube. On a 1.5" dia tube, it is approx 4.7" around (Pi x dia.), giving you 4.7" of weld connecting the ends to it. On a 1.5" x 2" rectangle tube (like our G-body arms), there is approx 7" around (1.5 + 2 + 1.5 +2), minus a hair on the slightly rounded corners. So that is a LOT more area to weld, making the end connection about 50% stronger.
Then the welding process can also add even more strength & integrity there too. We put the biggest weld possible there to give it the most strength & tie it together the best. It is obviously cheaper to weld less distance & put a smaller weld down, and that is one reason it's done. Plus we have seen many round tube "cheap" arms bend up on faster cars, but never seen a rectangle tube arm bend up. On our non-adj uppers, the rectangle tube also allows the weld to get right out to the corner of the "U" bracket end that goes on the upper ears on the rearend. This adds a HUGE amount of strength, since the uppers are trying to be pulled apart. We have seen top name chassis builders using round tubes, and either the U bracket pulls & stretches out & lengthens, or the tube breaks off from not enough weld attaching it.
And sorry to make this more confusing, but any round or rectangle tubing can be made/purchased in raw form with EXTREMELY varying ultimate and/or tensile strengths & other characteristics. So for any "general" statement of round is stronger than rectangle (or opposite) probably isn't very accurate. Basically it boils down to:
1) Round is usually lighter due to the less length of cross-section (not thickness), which is due to less weight per foot.
2) Rectangle tube can allow a LOT stronger weld to the ends to be made, but costs more to do in time & materials.
3) Stronger grade tubing (in round or rectangle) costs more too.
4) Either will probably work fine on slower street cars.
5) Probably most true and most importantly, and we all have heard it a million times, YOU GET WHAT YOU PAY FOR.
Unfortunately, we are all on budgets & usually #5 decides what parts get added to our cars. We are actually finalizing a new line of fully adjustable uppers & lowers for race only that will be way better than anything out there, but also probably be more on the high end of the price scale too. There is just no way to make super high quality parts with top grade components at low costs using USA parts & building them in the USA with USA pride & craftsmanship & customer support. Just one rod end on these at our cost is more than 4 on some of our competitors version. Again, you get what you pay for. We are also looking into a more economical version of everything for the lower HP guys that don't plan to go 10's & faster & just want something better than stock at reasonable prices. There is a need for that level & a place for those kind of parts, and we should be able to build a better "mousetrap" than our competitors & be very reasonable in price too.
Just always compare VALUE on parts, not just price, and expect the fine print many never tell you to be what you are missing for the reduced price. We could always purchase raw materials of a lower grade steel that nobody would ever notice & save money and/or lower prices, but we choose to make a better & stronger product, without going "titanium" crazy
Hope that covers it & helps!!! This is really just the tip of it all (if you can believe that!) The little (rarely mentioned) details of every little part of the materials, processes, components, procedures, etc, etc, can make HUGE differences in the quality and integrity of a product.
