here's a good link regarding the 'myth' behind back pressure:
linky
in a carbeuretted engine, back pressure results in less air moving through but the same amount of fuel delivered, so you get a higher fuel/air ratio and more 'bang'. if you reduce back pressure and you get more air moving through but a lower fuel/air ratio because a carbeuretted engine can't modify the amount of fuel to react to this; less power is developed.
a computer-controlled engine should be able to react to reduced back pressure by increasing fuel flow *provided* it is smart enough and the fuel delivery system is up to the task.
one would *presume* that the boxster is up to the task, which begs the question of why some folks find reduced low-rpm torque with freer-flowing exhaust. even the dynos published by NHP show a slight low-rpm torque reduction:
linky
all i can surmise is that it is a flaw in the dme mapping which could be corrected with a proper tune. it can't be an issue with the fuel delivery system, as the system has the capacity to deliver lots of fuel at higher rpm. of course, cams and valve timing should also be included in this conversation.
with respect to the air intake, the math is 3.2 litres x 7200 rpm = 382 litres/second. this will give you the size of the intake required to deliver the air at atmospheric pressure. any smaller than that and the engine has to do work to draw the air in (accelerate it to the speed at which proper flow can be provided). i did the math before and i *think* the number was 92 mm diameter.
there are a number of bottlenecks in the intake system that will cause this work to be done - the snorkle, the airbox, prehaps the intake manifold itself, etc. the actual acceleration of the air around these bottlenecks, followed by the deceleration afterwards, is a mechanism used to sound baffle the system. so, any work done to reduce these restrictions should be of benefit.
regarding the throttle body itself, while the increased diameter is a good thing, note how cross-sectional area changes with actuation of the throttle (ie, how much air gets through); it is not linear, so what a larger throttle body does, in essence, is create a twitchier throttle - more air (fuel) sooner, better 'throttle response' - an expensive sprint booster.