well, since a chain link is always the same length, increasing the number of teeth would also increase the diameter of the pulley. with such a short cage derailer I'm just wondering if you would run into problems with the chain between the pulleys themselves, or against the cage of the derailer. even a millimeter or two might do it.
I have no idea myself - just thinking as I type.
EDIT: WNG beat me to it (and seems to actually know what he's talking about).