I have used mine with and w/o water in the pan. Water in the pan takes longer to heat up, but is more temp stable. No water in pan heats up faster and gets hotter since you're not heating the water and everything else, but requires a slightly closer watch to make sure the temps don't run away from you. Just my experience.
Beware the wrath of a patient man. 18.5 WSM and "Buddy", my converted Budlight OT grill.