From what I can read, a WRX tranny should be good for the job so long as you plan on keeping the stock turbo. A LOT of owners who have done minor turbo upgrades (as small as a VF39 from STi) are reporting tranny failures from the extra power. So basically...
Stock Turbo:
RS tranny (may not be compatible with FXT diffs/driveshaft/axles)
WRX tranny (should quench your thirst for manual, and be compatible with FXT diffs/driveshaft/axles.)
Upgraded Turbo:
STi 6-spd (Expensive, Requires R180 rear diff, STi driveshaft and axles = more $$$$)
PPG Gear-set for 5-spd (Not cheap either)
One thing to keep in mind about WRX trannies: they are strong and can last-long when driven properly, but they
cannot take a beating. Not grinding/forcing gears, not jerking/stressing the gearbox, using Uncle Scotty's Cocktail, rev-matching, etc, are all ways to improve the lifespan of a WRX tranny.
If you beat it like a redheaded step-child, it
will fail sooner or later.
I'm not 100% sure on all this info because I just did a quick search, but it's accurate and it should help you figure out which path you want to take, or at least give you an idea of info to search for.
Maybe our resident FXT-owner Five0 will chime in with some infos...