The best bang for the buck mods are (roughly in order):
-X pipe with secondary cat delete
-High octane fuel
-ECU software (perhaps matched for the above mods)
-Titanium cat back exhaust (weight drop and small power bump)
Some might argue for a high flow air filter but I'm not convinced these actually add power.
With these mods you can count on 30-35 hp gain for about $6k. This car will leave any FD ratio modded stock M3 in the dust.