I did stillen high flow cats and megan racing catback exhaust. Definitely gain on upper rev band, plus it sounds awesome. About $1k all up for parts. CAI? its a waste of time. our airbox design is similar to R35 gtr so its very effcient, barely gains on dyno. Grounding kit will let the gear change abit faster thats all, i would go for Trans Go kit instead. Uprev tune is the biggest power play for n/a application, good tune with after market headers, exhaust you will see a 25-30 whp gain.