I just put them in a lathe and take the fins down to the highest point on the face of the flywheel. Not even balanced them, not had a problem getting the revs I need. No need to get carried away with trimming the fins right off or anything like that (they add strength!)

Given the nature of a cast flywheel and the limited machining (try comparing some of these flywheels) you'll note they can vary quite a bit. Sure, balancing may yield some more power but so will aligning the prop with the crank pin (which is another controversial subject).

BTW has anyone had a modified flywheel explode yet? Interested to know in any cases to get an idea of how statistically probable this is.