Cobb tested the hell out of that intake on the LGT and STi. Neither is exactly a WRX but it is interesting reading:
LGT
STi
It has been a while since I read them, but their conclusion is that it is a waste of time on the LGT, and a very minor improvement on the STi. I think they decided that it fared better on the STi because it has a bigger turbo, so the stock intake was a slight restriction.
Based on this I would estimate that on a WRX it should be considered a cosmetic mod, not a performance mod.