Originally Posted by wile e. coyote

Si3N4 has good fracture toughness I believe, and probably good surface roughness as well. Ball to ball friction would be low, and would not wear like steel on steel. I think mountain bikes would benefit more due to greater dynamic loading. I think even the cheapest ceramic bearings would be fine, cause if the factory can manage to even make the dang stuff, then what you're buying is better than steel.
As for the 'investment' factor, I'd say you're doing better getting good bearings than by gram counting. A bike is just a way to connect you to the wheels.
Now if you could get a bearing race made of partially stablizied zicronia, that would be the bomb.