Fill out your profile first off.
Secondly, read UNABOMBERS MANIFESTO, it's a handy sticky located at the top of the Newbies and FAQS section.
Thirdly, how much power it can handle comes with far too many variables to answer. Your goal is 300 whp for 1000-15000 bucks, easily done. Research 4 times and buy just once.
And finally, if you live so close to a performance shop for Subaru's, why not give them a call and tell them your goals? They will give you the exact way to do it, and for a bonus you can ask about your problem and get a good answer.