On my newest build (HT 66cc) I sanded down the raised circular lip on the head so that the mating surface was completely flat. I compared a new head with a stock head gasket, and that raised lip seems to be roughly the same size. So the way I figure it is I remove a head gasket worth of material from the head, which equals not using a head gasket and not removing any material (make sense?). The only difference is that more heat is being transferred to the head because of the flush mating surface.
I don't have a tool to measure compression (would like to get one), but I was overheating going up a 9 grade hill for about 1/4 mile (with shift kit). There are multiple factors at play such as plug heat range and oil ratio, but I ended up adding an extra head gasket to in effect bring the slant head back to stock size.
I've had two bikes overheat and seize up on that hill when I removed the head gasket or material. In retrospect I was using the either the stock Chinese plug which is pretty hot (equal to a NGK B4HS), or the extended tip NGK BP6HS which is 2 heat ranges cooler but increases compression further because of the protruding tip.
I guess what I'm trying to say is that in my experience so far (minus trying a cooler plug), removing a head gasket from a 66cc HT slant head will cause it to overheat with extreme load (huge hills) or long full throttle runs. I don't know much about motorcycles or high compression heads, but am I correct that they are meant for short distances of WOT, and NOT for climbing up very steep hills?
I just ordered some NGK B7HS plugs which are the coolest I will have tried. Hopefully this should help some. Also because I have a short billet intake, the carb was heating up and the gas bubbling (boiling)- hard to believe I know, but it's happened to someone else (search). A regular intake manifold would keep the carb cooler, but the ported short intake has little area for heat displacement.
Also, what octane gas are you supposed to use in an increased compression engine, and at what level (for example #91 if your compression is over X)? I like the idea of increased compression but I need this engine to last.
One last thing... I have read that these engines aren't built to tolerate increased compression (bearings, imbalanced crank, etc.). Even if you get it running well heat-wise... won't it cut the life of the engine?