As far as the Kyosho GT cars, you don't have to go "hog wild" on aftermarket options to keep the car competitive. The main thing is upgrading the 2-speed transmission to the shoe-style if the car has the old finger-style. You want to upgrade the brakes to the thick car pad-looking shoes from the flimsy paperish-type shoes for better stopping. When you do this, get the Losi brake springs to put between the pads (on the pade holding bolts) to keep the shoe from wearing them out when the brakes aren't engaged. The stock plastic shocks work fine but to fine-tune, you want to do like Jspeed said and upgrade to the shock pistons that are used in their offroad buggies and truggies. I would suggest upgrading the center front/rear drive shafts to the CVD ones. You can go CVDs on all four corners (which is costly) but you can get by with the stock dogbones. I still use the plastic chassis braces but have seen many Kyosho GT racers upgrade to the King's Head ones to better stiffen the chassis. Definitely get the front/rear sway bars and you are pretty much set.
