You still only have a fixed number of watt hours. A magicshine 4 cell pack gives 3 hours of runtime, so with a single cell, you'd get 45 minutes assuming a 100% efficent voltage doubler. Also it would be harder on the cell since you'd be drawing 4x as much current out of it; for a light that wanted 7.2 volts at 1 amp, if you ran it off a 3.6 volt cell you'd have to draw 2 amps, whereas with a 4 cell pack, each cell would only be delivering 1/2 amp.

That not only is not as good for the cell, but it means that you would not actually get the 45 minutes, because you must de-rate cells when you draw higher current from them. You may get 2 hours of runtime drawing 1 amp from a cell, but only 50 minutes if you're drawing 2 amps.