I would just buy both the SP-6 and SP-9 AND that 1-cell extender. Then, the choice is yours: especially for 2-cell application, they have FOUR different lamp assemblies! All I can say: the ES-9 really surprised me, how bright it is with just 0.85 Amps of current consumption. Here, you can really choose. Want the brightest and runtime doesn't matter? Just take the EO-9 for your 2-cell setup. For the SP-6 there are 2 lamp assemblies with a voltage of 3.7V. But you could also drop in two pieces 18350 batteries into the SP-6 and share / use the same lamp assemblies! Take the ES-9 for the 2X 18350 batteries, and take the EO-9 for two 18650 batteries in the SP-9 plus one extender. Or just take two pieces 18500 and just drop them in!
Well thanks.. Now I'm getting one.. or two... I was literally just thinking, where can I find a new Incan light, I've been really kicking around picking up some used ones on ebay but now that I know I can get new ones from lumens factory I couldn't be happier.