I have the vented Robinson racing flywheel on my Nova Mugen Mt-12 NMT and it doesn't slow down the engine rpm, that's for sure
It's of better quality than the RPM.
I buyed it for look, pretty cool if you ask me... But also for the fact that is one of the lightest flywheel you could find.
Anyway, some said that it could blow dirt in your engine main bearing, which is not a good thing off-road, but I didn't really happened on mine.
Only my 0.02 euro
DFF