320 to the wheels isnt that bad at all. Many guys get about that and are pleased. Depending on driving, you really wouldnt have to upgrade the tranny, or even the engine. I have seen (on NASIOC) a few 02-05s running 400+AWHP with stock internals. The power curve looked liked it sucked, but it was holding up. Just start simple, TBE, EM, maybe some sways. Get used to that and then move up. If it has been stock this long and all, you should go in steps or "stages". The questions you asked are common, and have been done so many times before. If you just read and search on any WRX forum and learn and understand the car, you will see where some of us are coming from.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX