Isn't the ring gear the wheel body (or whatever it's called), that is, even a fixed axle would have weight there, the gearteeth even provide stiffness. CV joints also contain unsprung mass, I'd say there might be a bit of a difference but nothing drastic. With modern fancy biomimetic wheel body geometries and everything you'll probably definitely be lighter than 80s steel rims. What happened to spoked wheels, anyway.