This is a controversial topic. My 14 Mil Cheetah, allegedly built by Dan Hull, didn't have a lightened flywheel on it (nor did it have straight cut gears.) Not sure if that was an oversight, or if parts had been stripped off of it, or what. I put some straight cuts on it, and left the heavy flywheel. I couldn't be more pleased with its performance.
Then there's the PVL, with almost no rotating mass....