Mine’s, a Japanese tuner, has done extensive testing on the GT-R. They have found that modifications to the intake (cold-air kits, etc.) are not responding well so far. However, Mine’s does have an ECU upgrade that should unleash some additional performance from the GT-R.

Mine’s has also developed an exhaust system and high-flow Cat-pipe. They have seen power increase by more than 20 hp with the complete system. Additionally, Mine’s has several suspension and braking options that should significantly improve the GT-R’s already stellar track performance. Stylistically, Mine’s has replacement body parts available in carbon fiber including a front splitter, canards, air intakes on the hood, and rear wing.

Other tuners have products as well. MCR has created a steering wheel hub with carbon fiber paddle shifters. ARC and Amuse Tuning should have exhaust upgrades available soon. All of these products should make it to the U.S.

Stillen has been racing a GT-R recently and developed a bunch of cool parts for the car including exhaust, suspension and brakes upgrades. Maybe check them out.

Also, there are some good blog articles here on Motorator about GT-R parts. Check the links below.