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if you are tuning for midrange torque you want an intake that gradually decreases in diameter. this serves to accelerate the incoming air charge and facilitate the filling of the cylinders on the intake stroke.
if you are tuning for max hp at max rpm then you want as little resistance as possible to the large volume of incoming air.
street cars are tuned to be useable at low and mid rpms, and on you boxster you will find an intake system that decreases in diameter as you move from airbox to throttle body to the plenum. the gt3 is a track car designed to live at high rpm. as such the intake system is as wide open as possible.
you can put a gt3 throttle body on your car, but it only means anything if you have removed all the other restrictions from your intake system. otherwise the air will restict at the airbox, then expand when it reaches the throttle body, then constrict again when it hits the plenum. that's a lot of useless work that your engine has to perform.
a larger throttle body will also change the performance characteristics of your throttle. think about the 1/2" tap on your sink. say it is attached to a 1/2" water pipe. open the valve halfway and you have half flow. open it fully and you have full flow. now put a 3/4" valve on the 1/2" pipe. open the valve halfway and you get 75% flow, open it two thirds and you get full flow. so, power comes on sooner with a larger throttle body.
the real question is - say you have a 3/4" pipe with a 1/2" valve - even fully open the valve isn't letting all the water through - did porsche leave any power on the table with the 986? jake says not in this case.
sorry for hijack.
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