hey guys i am in desperate need of help!!

i have an 07 gxp, and my dad has an 07' corvette c6
what are some of the best ways to upgrade the gxp for best horsepower benifits
i have already done the rod mod, bypass solenoid mod, bov spring and a homemade air box upgrade with a drop in k&n filter. i am now thinkking of an intercooler and exhaust. believe it or not the gxp isnt all that much slower than the vet right now until we hit about 130mph. can someone steer me in the right track, i can do the work but i am not very fimiliar with the 4 cylinder world yet because i work for cummins on big offshore boats.
any help would be appreciated