The times I've used an SOB1000 to drive an SSC P4 U2xxxH-bin, I measured slightly less than 1/2 amp draw at the tailcap using two Li-ion cells. That translates to an estimated 0.5A*7.6V=3.8W of power input to the SOB to generate about 3.5Vf*1000ma=3.5W of power to the LED. That's only about 0.3W of power wasted as heat by the SOB. Even if you want to be conservative and estimate that the SOB generates 0.5A*8V=4W of power, it is still close to 90% efficient. An SOB and an SSC P4 is a cool-running pair.