They are nowhere near as precisely made as A/C bearings, although there are new precision variants being made. They get hot at higher speeds, too, due to the fact that there is a large contact area on each roller and that the roller is turning and sliding at the same time. The bearings may do 7k rpm, but they will get very hot and you'll end up with issues like toolholders sticking, etc. due to the heating of the spindle. Tapered roller bearings are awesome as ballscrew support bearings, though. It saves a ton of money to use them for that.