Well my origional setup was the TTP headers, high flow cats and Gemballa Muffler, and yes I do have an ECU upgrade that was done on a DYno at every breaking point up to 160 mph. I think the HP gain gave me about 45 HP. I still have all of the same setup except I decided to upgrade my muffler to the RUF model coming out of the bumper. The Gemballa of the same likeness was WAY too much money for me.
I am still working on putting together my twin turbo fund
Yes I do feel that the TTP header in combination with high flow cats are a must for these cars. THere was a very noticable difference in ower and sound after installing these parts.
and FYI the exhaust resonance levels are very acceptable by my standard. I bought a Porsche to listen to the engine and exhaust, not the stereo system