We used to race Supermono (single cylinder in road chassis - generally RGV chassis with XT600 motor) and we used high comp pistons, and bored out as well

Now, I don't know about the XT660X motor, but we had to run avgas (otherwise would blow head gaskets) and would crack crankcases fairly often - about every 18 months. But they went like you know what!

Also ran different camshafts.

As I said, don't know about the new engines, especially in regards to fuel injection over the carbs we used to run.

I reckon reliability would be compromised with a high comp piston.