I've had an ally flywheel on my 748 for about a year now.
no downsides, still ticks over well, doesn't stall, engine braking is stronger and the thing revs up quicker...........
What could happen is that the reduced inertia could exagerate a poor fuelling setup.
Have to agree with Shazaam on all his points
Cost about a £100, fitted in a couple of hours comfortably. I think it's one of the best mods you can do