Disclaimer I am far from and expert in real world experience but with aluminum heads 10.5 compression is nothing _if_ you have done your homework on the quench and cam. It sounds like that is what you are leaning to. Get the quench right, nice street-able cylinder pressures with the cam and aluminum heads. My bet is that is the right decision. My KTM 500 had a 15:1 compression ratio so it is not impossible.