I would think volume in the intake would be important for lower rpm power as you are not in boost yet. The factory sure does have a larger pipe from the TB to the filter. Most boosted race cars run larger pipes. Although it will increase lag a little. Also note how the outlet of the turbo is actually "tapering" out from the turbo. Maybe the tapering effect means something. If that's true that your piping doesn't need to be any bigger than the outlet of the turbo, then you might as well put a smaller TB on too.

 

i run 2.5 all around... i used to run 2.25 from turbo to ic then 2.5 from ic to tb but i changed it up a little