Your Stella is exhibiting that behavior because it is regulated. If you're seeking the highest luminous efficacy, look for lights that have the Cree XP-G LED. These produce the highest amount of light per power input. As reference, per
Cree's spec sheet, the XP-G LED is rated to produce about as much light as your Stella (120 lumens) at about half the current (350 mA), which means you'll likely see almost double the runtime.
But lumens isn't really all that matters. Like the MHz/GHz wars in CPUs, lumens isn't really a good measure of how useful a particular light is. It is useful in comparisons between similar lights; but to be useful to a cyclist, the LED must be paired with the correct optics that can shape that light energy into a useful beam. For example, a concentric and symmetrical beam may be useful for mountain biking where you need to watch out for low-lying tree branches; however, it is less useful for general road riding where an asymmetrical beam with vertical cutoffs is better because this beam concentrates the light energy lower and onto the road.
Try browsing the
Candlepower forums. There's a lot of good information there if you're a light geek.