Yeah, the output shaft bearing support is a good idea.
As far as the theory behind the billet mounts..the rubber in the factory mounts allows the entire engine to move, and the solid mounts don't. There is nothing that has to "give" with the solid mounts, assuming you have the bearing support. All power is directed toward the back sprocket, rather than on case or output shaft flex. Just my 0.02...for what it's worth.
What kind of axle are you running? Are you 100% sure that the rear sprocket is lined up with the front? This is cause of most front sprocket issues, I believe.