Parts for these cars are pretty expensive. Longtubes are $2500, exhaust is around $1300, tune is around $500 and all that won't gain you too much... Not to mention how much of a pain in the ass it is to install these things. The v8 is crammed in there pretty tight meaning no room to work.
Also, several common failures.
-Water cooled alternator $525
-A/C compressor $900
-Starter $200
-Flywheel $900
-Power steering $depends
-Valve cover gaskets
-Window regulator
-etc...
I have had mine for quite a while and I have been affected by the valve cover gaskets, window regulator, a/c compressor and the power steering
Despite all of this, this is probably my favorite daily
Good place to read below
http://www.audizine.com/forum/forumdisplay.php/46-B6-B7-S4-RS4