Mosc is pretty much right. I know a guy running the APS 3.5" catless on a VF22 on his 06/07 STI. It's too big even for the largest IHI turbo. Another guy I know is running a 3.25" catless, don't remember what brand, on his 06 WRX with a VF34. It was perfect for the car, it ran 11.8 at the track. I'm running a 4" to 3" ERZ catless bellmouth on a VF34 on my 03 WRX. I'm running mid to low 12s.
Unless you are over 500hp, you don't need anything larger than a 3.25" on the 2.5L and 3" on the 2.0L. Well on the 2.0L, the block really isn't reliable after 400hp.
VF34'd Corvette Killer aka Doug