i dont know if your cossy has been lowered or not but under boost they all squat!!!!...i had a saff that was a nightmare..sparks everywhere when i floored it.. if it has been lowered and you frequently carry passengers you should put it back to standard height...if not uprate the springs..ride will be a little harsher though....as for the exhaust etc..fitting a cone filter and exhaust will gain 20bhp..but you could get 270 on standard anyway..depends on what boost your running

my sapph bottomed out that much i wore through the diff casing!!!!!!!!!!!!