I've never liked the idea of machining pistons. You just dont know what stresses you are introducing, or what sort of strength you will have. Especially if you a worried about pre detonation.
As echo says, run the stock compression and always use 98 ron (I've heard rumors that mobil 8000 is better than Optimax).
Get the car dyno tuned so that you are sure your not getting pinking, and you will be fine.
Iv'e also heard of people using two head gaskets together, or a thicker head gasket to reduce comp.
