Turbo header horsepower is but one of the criteria in choice, they have to fit, they have to last, especially at the temps and PRESSURE we run them at. Keep in mind, you can't get something for nothing, folks often mention that turbos make "free HP" from the normally wasted exhaust flow.
In reality, the back pressure in the exhaust system before the turbo is from one and a half to near twice the boost pressure seen in the intake!!!
I have seen some scary, piece of crap, home made turbo headers! But by getting the exhaust into the turbo, by any means, they will make more HP than they did unblown.
A freer flowing, well transitioned header not only minimizes the back pressure in the exhaust port, but retains a higher velocity (more kinetic energy) to spool up the turbo quicker, and usually to a higher speed, until the waste gate opens.
For those of you without a flowbench, a few simple tests can tell you how well the turbo header works.
Route the EXHAUST from a 5 or 6 HP shop vac thru each exaust port one at a time (make sure you make the flow straight into the tube in the same direction the port flow in the exhaust goes.)
Now put your hand over the other ports on that header, a well designed header will have a suction on the other tubes.
On our passenger side header, route the flow directly into where the crossover comes into, connect the crossover tube to the header in the same configuration first. Blow the air in and feel for any suction in the exhaust ports. Suction in the other exhaust ports is good! More is better!
In many poorer designs, the rear, passenger port gets pressure blown backwards INTO the port! This is a nasty restriction for that cylinder, causing a noticable HP loss there. This can cause that cylinder to run rich due to residual exhaust remaining in the cylinder. This can also happen to a lesser extent in the passenger side front port. Also, any header that dumps exhaust at a near 90 degree angle to the runner, also restricts the runner flow too.
By doing these simple tests, and analyzing several designs of headers, its easy to see which ones were designed on a flowbench, or not.
Just some random thoughts by an old guy that has been turboing cars and bikes for over 35 years!
TIMINATOR
P.S. many have run crappy designed headers on very fast cars, so now ya gotta question how much faster they could have run! Just sayin'