Currently I have only changed my center muffler with twin manta hotdogs, and upsized that section of the pipe to 2 1/2 inch. Cost only $360 bucks fitted and gave a slightly nicer note and economy is better but only just, defenitely drives like a better motor. Even wife noticed it drove better and thats saying somethin!
There is alot on here about the CAI kits, Prices vary but around $600 for ss including Pod filter, battens also does one and it sounds like it has good improvements +-10rwkw. i am getting this done as soon as the TAX Cheque comes through along with the pacemaker Extrators and possibly the SS 80mm throttlebody. Some blokes have also fitted the Helix spacer and battens does these along with herrod, not sure on price but sure someone will fill you in. there is big raving about the underdrive pullies and apparently for about $750? they also give good improvements but have read some guys having clutch probs??
This is only what I have seen from reading the feedback on the site, but most the blokes sound pretty happy. Hope the input helps, check out the signatures on the Gregory's FPV Forum of some of the guys and this will give you a good indication of what they are doing to the boss motors.
Hope the info helps mate, enjoy the Beast!!