How common is it for people to use such a fine pitch on a router used to cut aluminum? I understand that the main tradeoff for using fine pitch would be a loss of speed, so would it be worth it if I could find a ground ballscrew with a pitch this fine?

thanks

Speed versus the ability to vernier adjust position to a finer degree is the choice you're faced with.

It all depends on your accuracy requirements as to whether you want 5 versus 10 revs per inch of travel.

With a fast enough motor, it won't make any appreciable difference. Moreover and more importantly, your ability to plow thru the material will depend on how fast you can cut the metal, not how fast you can move the spindle in the X or Y direction once you make the cut ....