depends on supporting mods....which stage?....are you talking about from just OTS maps?....or are you saying EM in general? You can make more with a Protuned map as well vs. OTS
Unless it was swapped before you got her. 2003 wrxs come with a catted up. I want to say that with just a tbe and AP cobb estimates around 230 whp with the stage 2 map. I don't know how much a topmount would add, and a protune if you were going for that. I would guess somewhere in the 240-250 range?
I would highly recommend getting rid of the cat in the uppipe. The last thing you need is that thing breaking up and getting into the turbo, and having a otherwise modded car just makes that scenario more likely. The ebay top mount won't help much with the generic Cobb maps, but should give you an edge for protuning. I would guess around 230whp overall.
The highest I've seen is Unabomber on NASIOC and he's pretty much done everything you can do with a stock turbo, and he's just shy of 250...
your actual numbers will vary depending on which dyno you run on... so don't get too hung up on absolute numbers