It's not the rotating mass, it's just the mass. The engine itself weighs 75 pounds, and it's 2 feet off the ground. If the gyro effect had anything to do with it, you should be able to run the RPM up to about 5000 and put your feet on the pegs at a standstill. But you can't.
I'm no engineer, all I know is with substantial time on both the 350sxf feels substantially lighter than the 450sxf even though it was within 4 lbs, whatever the reason. The husaberg 450 I owned with the 70 degree motor felt lighter in motion than any 4 stroke I've ever owned (unless you had to pick it up) even though it was the heaviest by far so there must me some truth to it. Isn't that why Yamaha has the backwards slanted motor? To get the moving parts closer to where they effect the handling the least?