Stock gtp suspension is great... the best trick you can do is try to get some bilstines on there off a gxp. The key to autocrossi g is desgining your car for the slowest class you can... commonly you will find running in a stock class with every possible mod you can do is easier than running in a modded class with only a few of the allowed mods.
Hands down the best mod you can ever do is tires. Get slicks (hankook c71 is the most popular) and run the most mods you c can for the lowest class that allows slicks. If you want to stay on a street tire get as wide of a autox tire you can... r615k direzza z1* or zii. Toyo ra1. Among others in the 200 treadwear catagory.
