It isn't that complicated - I have always been under the impression that 150hp/litre is pretty easy from a modern Japanese turbocharged car. So don't feel too bad if he has better power/weight.
Thus, he probably has a minimum of 300hp - in fact the stage III bit could mean 350hp or perhaps a little more (tough to get more than that from a WRX without spending big $$$, I think).
Much the same as a 930 responds to a freer exhaust and more boost and colder intake, so does a WRX - it is just that the parts are cheap by comparison.
http://cobbtuning.com/wrx/powerpackage3.html
I note no power level claims. I had a quick look at their forum and they are insinuating more like 350hp.