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Test drive a used model - either at a dealer, or perhaps post for help on a regional forum (another forum member might give you a test drive). I don't think there are many here with direct experience of modded S5 and how they compare to stock M3's.

From what I've seen, typical bolt-on mods for the M3 will net you around 450BHP and perhaps 350 ft-lbs (maybe more torques). The best bang for the buck comes from xpipe/software/pulleys - the exhausts posting significant increases are also quite expensive. (Not sure intake does anything).

There are quite a few threads on the subject of bolt-ons - should make for some interesting reading.

I am not sure if I'll mod mine for power, I think it's pretty fast as is. I will probably do an exhaust, the M3 sounds incredible with a good exhaust.