Mike...that's exactly right...removing material anywhere inside the case will effectively lower base compression, so you have to tread lightly.

When I mod a sleeve for example, I basically just radius the sharp edges in the fuel/air mixture's flow path. When I do a crank, I'll bell mouth the opening of the induction passage where it opens into the case, and polish the passageway if necessary...and sometimes add silicone in the area immediately under the carb. I will add here though that I don't race, and just do my own mods for bashing, so I'm not looking to get the max out of any one particular engine, but basically to improve efficiency somewhat.

If you go and remove a lot of material from the crank, sleeve, case, etc. and increase the internal volume by a substantial percentage, that will lower the flow velocity, soften the bottom end torque, and you'll need to work the ports and timing for top end power to get something back, or you'll end up with a doggy, less powerful engine than you started with.