Its been a little while since I checked this one, great thread, gobs of useful info, very grateful. I would like to add, something I did whilst I did the rebuild, and although this is definitely not a weekend project or anything 99% of us are qualified to do, it is a recommended mod, if you have the extra scratch and or the opportunity ie: the top of your motor is accessible. When I did my motor rebuild, I had already planned to powdercoat the valve covers and intake manifold, along with the motor overhaul and upgrade (camshaft, pistons, bore, etc.).
I, just like most other newbies, was becoming aware of all the modification opportunities possible on a hotair GN. This one is just simple motor math. Allow the air I'm cramming into the motor to flow more freely so I can cram more air. I delayed the rebuilding of the top end of the motor a little so I could put some cash toward porting and polishing the heads and the intake so I could squeeze as much horsepower out of the motor itself before tucking it away in the car and hopefully never having to touch it for many years to come. I can say that I'm glad I did.
If you have a good, qualified, individual perform this work for you, and its done properly, you will squeeze and extra 20-25 hp from the motor alone. Which is a huge gain given the limits to what can be performed overall, and the given limits of the motor itself. Now bear in mind I had my stock heads and intake done for around $800 bucks. This is the low end of what you would expect to pay to do the stockers. I personally would recommend, if you were seriously considering doing this on your car, to save the extra cabbage,(around 2k) to purchase the aftermarket aluminum heads from champion which are of course expertly done and either sending in your intake to be done (champion will do this as well I believe) or purchasing one second hand privately. Or of course the option of purchasing one new. Many dealers associated with this site, and members alike, have these items easily available for you. Obviously a project of this magnitude requires you to do your do dilligence, but everything can be easily found.
So the moral of the story is, if you happen to be working on some or all of the outer mechanicals and pretty much have relatively easy access to the top end of the motor, and you want to find a few more ponies. Seriously consider spending the coin and having this done. Not an easy mod by any stretch of the imagination, and not one that is readily seen or noticed by anyone, but certainly something you will feel, and ultimately feel good about knowing, that your getting the most out of your little 231. I know some of us are here to rip up the track, some to cruise and show, and like myself, street and drive it into the ground while demanding respect from ricers (I have an s4 too btw, that's not meant to be a jab). All of us have different wants and needs, and this fit right into both of those categories for me very easily.
Sorry for the war and peace post, please forgive me. I have and continue to learn so much from this forum, and for that I am eternally greatful. Hope this adds some useful insight and inspiration.