Right now there is no kit, but West Covina Toyota David Draper kit is coming soon. That kit is well thought out, got a lot of imputs from people who knows what they are doing. Thats the only kit I would recommand when it comes out.
For a GTS you can add a better header, Cold Air Intake
, Lightened Flywheel, piggyback unit to get more out of the engine (this is the piece that cost the most), and ofcourse stickier tires.
If you boost a GTS to 5 psi you get around 200HP (Blitz dyno shows this) and this is about the limit of the boost GTS can handle. I am at 7.1psi with room to go and I am already at 223.1 HP SAE and 198lbs of torque. Without modifying engine GT can handle more than a GTS.