Interesting discussion going on here. I have an 08 STI with a TBE and a stage 2 tune. It is putting down 340 HP to the wheels and 390tq. This is my second STI, started with an 06. I also just got a 2011 Spyder. The 2 cars are totally different, my STI is my DD and the Spyder spends most of the time in the garage covered. The STI is an in your face kinda car, it's ruff around the edges and just want to be driven hard all the time. It is the funniest cars I have ever driven, that is until the Spyder came along or along came the Spyder
. The STI handles very well and I love to drive it when the roads are wet, it slides through corners with total control.. IMO the high roof line makes it a little top heavy and at limit driving does under steel some. The Spyder on the other hand is the cut above the STI in every category , the most neutral, balanced, smooth car I have ever driven and that includes a GT3/RS. The handling is almost like the car reads your mind, it's like you move your eyes and it's there. Putting and STI motor in a boxster would be cool, you might have some fit issues with the motor and as you mentioned the trans, but I think the idea is a good one. That would be one bada$$ car. Good luck with it if you decide to do it.
pcw