K&N's intake has actually been proven pretty well for the GXP. It also depends on your budget and where you want to take the car. There are a few catback options, just search for them...Borla, Magnaflow, SLP...etc. Hogan makes a 3" Downpipe or Doug Thorley makes headers for the LS4 cars. People have also used various hand held tuners too, but best results will be yielded from HP Tuners...There are LSx rockers, LSx based cam swaps, LSx intake manifold and TB swaps along with the correct fuel rails, etc. Displacement on Demand will most likely need to be deleted to run an aftermarket cam. You can buy solid rubber replacement lower motor and transmission mounts and a poly upper mount for hold it all in place too.
