A few flywheel quotes:

"I had one in my old Cav V6. the most noticable thing I found was lower fuel economy on long runs. The thristy V6 round town was capable of high 35-40mpg on a run. This dropped to 30-35mpg after having the flywheel lightened as the stored energy in the mass of the wheel wasnt turning the engine "for free". Round town I didnt notice any improvement except in the drivabilty, much easier to get going and a much nicer driver."

https://passionford.com/forum/showthread.php?p=3392763