2 - Yes, SST-20 will get ~700 at max on 2.8A, XM-L2 closer to 900-1000. Very little difference to the eye in reality, except that the SST-20 is much throwier as you already know.

3 - Neither. Both those drivers run at 2.8A max, so the drain over the course of one battery’s charge will be basically the same. The XM-L2 will probably gradually dim sooner due to insufficient cell voltage earlier since it has a higher forward voltage.