Hello there,
Lower duty cycle has just the opposite effect: it lowers the efficiency. The highest efficiency you can get is with lower current not higher current even if it is pulsed.
To find out why this is true, you can look at the efficiency curves for a typical LED. What we see is at lower currents the efficiency is nearly a straight line meaning it is nearly constant over a range of low currents. But as the current increases more we start to see the graph curve more and more toward the horizontal, and a horizontal curve means very bad efficiency. So the slope decreases with current too and that means the efficiency is less and less. For example, if you take a normal 20ma LED and run it at 10ma, you get more light output per amp. Since the voltage also comes down, that means higher efficiency. So running TWO of these LEDs at 10ma each produces more light than running ONE of theses LEDs at 20ma. Running FOUR of them at 5ma produces even more light yet, for the same TOTAL current and also the voltage is less so the efficiency goes up even more. The limit to the effectiveness of this is when we get close to the straighter line part of the efficiency curve. Just how much difference it makes though depends on the type of LED.
So there are ways to deal with the efficiency, but they usually are not as simple as we would like to see. Buying more LEDs is the only way to get more efficiency so you can run them at half current, and that may not produce enough extra light anyway.
I have rebuilt one of my lights several times now. As the LEDs get better i have replaced the LED several times. I started with an early Luxeon Star 1 watt, then went to a better one, then to a 3 watt Cree. I run the Cree at less power though not only to get higher efficiency but less heat and longer battery life. I had considered going to a much higher power LED like 10 watt, but the extra heat is just too hard to deal with in a hand held light unless it is a big one and this one is smaller so it would mean total redesign.