You guys worry too much, just put a bunch of power down and worry about fixing it if / when it breaks.
Seriously though...The wrx tranny is weak, but not made of glass. I'm not sure what my car is putting to the ground, but i'm beating STI's, and running 12.8's in the 1/4 mile. So the wrx can definately be as fast as the STI (or faster) with minimal mods (you can look at my list) and a 2.0L engine and it _can_ do this on a stock tranny...for a while.
When we did my clutch at about 60k miles, we took apart the tranny to see how it was doing. Everything looked perfect, gears were smooth, shifted fine. I was exctatic, and thought like many others that the trans was fine, there were just a bunch of people out there who can't drive. I sent the gears out to be cryogenically treated (figured hell, while they are out) and two of them shattered. This means that there were hairline stress fractures in the metal. I bought new gears, had them cryogenically treated, and everything seems to be fine.
There is a reason everyone on here tells you that you need a tranny before modding your car, they aren't just being dicks. With mild modifications (Stage 2, maybe a vf 34) you can definately get away with the stock tranny, for a while, if you drive it right.
The unfortunate truth is this: If you have a modded wrx, eventually, no matter how nice you are to it, you will need a gearset replaced. Noone can say when that is, i have seen 350hp cars drive 60k miles on a stcok tranny...and i have seen stock cars break their tranny the first time at the track. Everyone has a story of "i knew a guy who drove this far on this setup", but that doesn't change the fact that our transmissions are weak. I mean hell, there can't be that many bad drivers among us. !Laugh!