I agree with Scott 100%. Modding a WRX to the point that the tranny can't dependably hold the power is a waste of money...
PPG also makes Helical cut gears which I don't think are any louder than stock. You can get them with either dog engagement or synchro too. They're the strongest helical gears as far as I know. I think most people just replace 1-4 and leave 5th stock. You can even mix and match. Some people will do 1-2 straight cut and 3-4 helical.
I've heard excellent reviews of
Andrewtech, but depending on the power levels you'll be reaching, STi RA gears may be enough for you and they're half the price I believe.