I always use the first pull or two to determine how much I can pull... If the healers mana bar barely moves, or the healer spends most of the time dps'ing I will pull more to accomodate the group.. Also, the pace of the dps'ers will impact this.. If mobs die faster than I can round them up, obviously I can pull more.... I usually dont do superpulls, but if I see that we are obliterating mobs and flying through the instance, we might as well pull accordingly...

But if people tell me to slow down I will, and if people tell me to speed up i will do that too(as long as I feel the group can handle it....)