I just changed from a stock flywheel to a raceseng s2 on a gx390, thats going from around 12lbs to 2.2 I think. I can get it to idle pretty nicely @ about 1250 rpm, could probably sneak it down lower if I wanted to. The only issue with the light flywheel is pull starting it is impossible, the inertia just isn't there to get over the kickback, before I could pull start it sitting in the kart. Smaller motors may not have this issue though.
It's an old one I got used its not an arc it's another one I can't think of who made it off hand... but I believe it set at factory timing and I'm not using a cut key... I just wanted to see if it was normal to want to idle that high with these light weight wheels