Since you are planning on turboing the car I would leave the automatic. The automatic has been proven to hold up in high horsepower applications. With a few inexpensive mods such as replacing the fluid with a good synthetic ATF and adding a good auxillary cooler the slushbox can be made even more bullet proof.
There are not a lot of options for the manual box without spending lots and lots of dollars. Dog gear sets will get you a stronger tranny but will cost big $$$ and street drivability will suffer in my opinion. Even the WRX tranny is questionable when it comes to strength.
Imagine the launches you will get with your turboed 2.5 and slushbox!