Just bought a 2006 G35X, 89k miles. Looking to do a couple upgrades, specifically intake, cat/exhaust. Had a few ideas for each.

Injen Race Division or K&N Typhoon Cold Air Intake
Stillen Exhaust System.

Unsure if I need the exhaust or can just switch the cat out to a Stillen high flow cat? And would putting a transmission cooler on this far into the life of the tranny be helpful or a waste?

Stillen products are good but expensive. Injen is quality but I don't like the position of their filters on the G. For a shortie I'd suggest R2C intake system (http://www.tunerzstore.com is a great resource and has one on his G's).

I'm running a full Invidia exhaust and love it (HFC's and full cat-back). Ark and others make great systems for the G as well. If you are looking for sound then jump on youtube and google "g35 exhaust" and you should find lots of people showing what different exhaust systems sound like. I've not heard anything really negative about the Stillen HFC's other than cost.

Never hurts to have a transmission cooler.