I find 3V incan output to be about the most used/useful output, so I run
1600mAh NiMh 2/3A cells in my E2e, with a couple magnet spacers, using Tad A3712. Great runtime and very steady output. I use other brightnesses, just not nearly as much as 3V. MN03 is still available and does fire on lower voltages. But Tad Customs sells brass E-sockets for his bipin Xenon lamps of various voltages, and thus is irresistible, though technically not "bomb-proof" like SF product, Tad's product does fantastic anyway. I like MN01, but very difficult to find. I use Lumens Factory HO-E1A with 1xAA, but recently discovered 2xAA with Tad's A6010 is twice as bright for twice as long, but still a level far below typical 3V output. I am experimenting with brighter setups, but so far anything above what Tad's A3712 puts out on a single IMR has been beyond overkill. I have a fistful of well-regarded High CRI Yugi 5mm LED that will work with 3V without a driver in Tad Customs socket (safely stored sitting in a package at a previous address) that I am anxious to get my hands on in due course. Tad Customs lamps for his E-socket max at A7212, for 2xIMR or 3xCR123A, 200lm. That sounds lame... but in practice the output is steady and it's bright... sometimes wow! bright! depending on circumstance. McGizmo has pictures on his site showing an E2e where he removed the bezel to add an o-ring to increase the distance from the lens to the lamp socket, allowing him to install a G2 halogen 1000lm lamp. fivemega sells custom E-series 1000lm incan 18mm tube lights. Keeppower now sells excellent high amp hybrid IMR18350 cells with high capacity, 1200mAh. I hope this gives OP an idea that though an LED drop in is one single option for upgrade, multiple upgrade paths exist and can be taken simultaneously... E2e, you know?