Yep, it does go quickly! You do end up trading off time for a bit with that method although you need to spend the time going through the 80 grit doing it as fine grinding. I would prefer making up a diamond wheel grinder than going to 40 grit for a small mirror. Take a concrete or tile (even the 3" ones are usable for this) cutter wheel and make a cup so you can bend it a bit and get the nut into the cup of the wheel and set the assembly onto a drillmotor or a general purpose motor and by tilting at a certain angle and rotating the glass underneath, you can do a fast job of hogging out very deep curves very fast. If you look around on the web, you'll see some stuff on this technique.