On my marble block I sanded the case mating surface for the cylinder flat too. I also smoothed out a couple areas inside the case below the transfer ports that have sharp edges and angles.

Are you guys talking about only gas engine counter balance cranks? Because there are plenty of examples of counter balance cranks that turn well north of 10K rpm in nitro engines. Some are ported thru the crank and some are rear carb like ducted fan engines. I've modified the one piece disc counter balance on some buggy engines before too. I have never broken a crank I modified. If they are built to take the rpm they will hold and if not you will know soon enough.

Reed or not the whole point is flow. I've made simple clean ups on 26cc gas boat engines and compared to stock it makes all the difference. They start easier and make a little more power.

Im not making any claims I'm just trying this out.