I'm not sure I'd want to rev an M20 to 5000 rpm ! But I'm completely with you on accuracy of trueing the flywheels, and of dynamic balancing. I had the crank in my 350 AJS rigid trials bike done when I built the engine about 10 years ago. I was surprised at how much material was removed from different places on each flywheel. Its done an awful lot of work in those 10 years and is still as smooth as ever. My WD G3l Matchless also got its crank balanced, and piston skirt coated when I rebuilt that one about 18 months ago. Again, a lovely smooth running engine. We now have the time and technology to build these engines to a much higher standard than when they were first built. I can't see a reason not to do it and enjoy the results.