I thought not all drivers are constant current, i.e. that direct drive (fet) are not constant current, hence the decrease in current and lumens as the battery empties? (Understanding the difference between Linear, Buck, Boost and Direct Drive drivers - Flashlight Modding and DIY Parts - BudgetLightForum.com)
Also linear drivers (such as those in the C8+/M21A) though heating away excess on hight, still averaging an efficiency of 89% over the whole run, I had understood. (ib.)
It was not clear to me whether the usually included buck or boost drivers were more efficient than that? I have not noticed any difference to that effect in tests with tail currents or runtimes so far, except that the XHP50.2 3V and the XPH70.2 6v do have better runtimes (is the 50.2 led provided with a boost driver despite its 3v?). However, their halving of the throw and their spill of lumens to the side makes perhaps for more efficiency loss than the lm/watt gain from the boost driver and the led?
These are the kind of questions that have been unanswerd in my head the past few days: what are the resulting efficiencies in the tradeoffs between throw, beam pattern, led/driver efficency, stepped dimming or 12mode, etc.