improve your high-end with the Injen CAI
exhaust is the 2nd most effective axle-back system.
TPR exhaust is the 1st most effective axle-back system.TRD
KAZUMA exhaust is THE most effective CAT-BACK system. (its not street legal)
DC or HotShot header dont do anything for the GTS. your best bet is to PORT and POLISH the stock header. (as Clos as done)
Your first appearance mod should be the s2000 Antenna.