I don’t use 14500 in mine anymore, there is little extra brightness to be had, the moon mode becomes too bright for my taste and duration is lost, but most importantly there is no energy density benefit compared with a good NiMH cell, at a fraction of the price.
I have several Efest 14500 700 mAh, bought in enthusiasm for a BLF/Kronos X5, that is the only torch I use them in now (and the only cell that works in it), and I rather think the form factor is not very useful, compared with e.g. 18350 or 16340 of similar or even higher capacity. It seems to be a dormant product, and also rather a dangerous one, if mixed up with AAs and put into something that cannot cope with 4.2V per cell (i.e. almost everything).
Also there is no low voltage cutoff in the S15 when using 14500, the torch seems really designed for NiMh, I have now spoilt two 14500 by letting them run down until quite dead. I would have been better using protected 14500 if such a thing exists with any decent capacity. Even if it did, it would not suit my X5.
I love my S15, but it runs exclusively on NiMH or even alkalines when travelling. It is an AA torch and maybe still the best one.
It is partnered these days with an On The Road M3 (16340 cell) which is much smaller, far far brighter with a nicer tint, but this doesn’t have the very low moonlight that I find so useful in the S15.
Actually the 16340 is becoming a staple for me, it also fits the 18350 torches and even my 18650 ones that have drivers that can cope with 1S or 2S, as well as being interchangeable with CR123A primary lithiums, for situations when disposables are still the best choice.