I don't think the 3.4L swap would be the only way. It's a little expensive.
Tell the guys with the Nissan Skylines with a RB26DETT 2.6L I6 that kick out OVER 600HP to swap out their engines for larger ones.
http://en.wikipedia.org/wiki/Nissan_RB_engine#RB26DETT
So I think it is possible to get a 2.5L DOHC engine over 300hp... correct me if I'm wrong.
http://www.roocksport.com/main/tuning_boxster.php