If you are using H cylinders with L heads then you're already on HRC SP spec, which is an avgas spec.

We have found volumes that work for unleaded 95 here in Thailand, and it's not that radical. Only if you have a high exhaust port should you consider a decrease in volume.

You'll find that a relatively low comp NSR revs nicely and makes smooth power. Up the comp and although it'll start building midrange at first, you'll then hit a wall where it won't rev.

Tightening the squish works, as does reducing the squish area, which slows down the MSV meaning less chance of the dreaded detonation at higher rpm.

If you're after more power then look at the areas that a production line doesn't take care of such as casting flashing, sharp edges etc.

When it comes down to it, the compression ratio is very much like the ignition timing. It doesn't need to be right on the edge to be good, but on the flip side and erring on the safe side can yield a nicer ride. It's also very easy to go too far and undo all your hard work in a pool of molten aluminium!