a throttle body on its own will not do much....
you can add about 15% plus increase in hp over stock with the following combination:
-cold air intake AND 73mm throttle body
-performance exhaust, catbacks / x-pipe
-re tune of engine management system with tuner (diablo predator or equivalent)
you can search out these parts and details at
(lots of other suppliers, lethal performance, summitt racing, more)
together these will provide a noticeable gain in your GT, individually not much.
additionally i suggest you swap in 3.73 gears in the rear end, will make you much quicker from the get go.
this is about the limit of "standard modificatiions" - beyond that for serious power your options are forced induction (supercharger or turbocharger), or installation of a nitrous