SRT in the US makes a nice twin turbo kit for the GS 8 cilinder engines.
Those will put down between 400 and crazy horspower at the rear wheels.
Toms make some nice clean spoilers and good chassis mods (good upgrade when shooting for 550 hp).
Tein has some nice coil over suspension setups. they have an EDFC module that you can use to alter the damping.
You will need bigger brakes, AP has a big disk 6 pot conversion.
Bigger brakes need larger wheels, 20's look nice. Get some custom made ones like HRE.