[Aussie Mike]

Fly cutting is usually done to increase piston to valve clearance. When installing a more agressive camshaft with increased lift and duration there is more chance that a valve will come in contact with the crown of the piston with disasterous results.

Flat top pistons often don't have a valve relieve in them so a fly cutter in a milling machine is used to cut a half moon into the top of the piston under each valve make clearance for the edge of the valve as it opens.

It's probably not a good idea to cut too much material out of the piston crown to lower the compression as this will weaken the piston. Dished blower pistons have a lot more material in their tops for the dish to be machined into.

To drop your compression I would suggest a set of thicker head gaskets or finding a set of heads with bigger chambers.

Cheers