I've stuck 3.7v lithium-ion rechargeables (14500 and 10440) in LED Lenser lights.
These higher voltage cells don't blow the emitter. In fact, the emitter tends to produce more light. However, these cells DO tend to blow the boost driver. Basically, if you run a 3.7v li-ion cell you may no longer ever be able to run 1.5v AA cells in that light. The light will probably still work, but only with the higher voltage cells.
I've heard this before as I looked at an M1 a while ago and a few people said much the same that once you've gone to the higher voltage there's no going back but if it doesn't blow it .................. and this is the big if ........ the run times are half decent with higher voltage batteries I'd go with it but would really want a couple of hours.
I have one of the little Q5 Sipik SK 68 lights which has a very narrow focus beam and is as bright as I need it to be but only has about 40 mins run time ... I'm after something that offers the same / a smidge more power if possible, I'm using it with protected 14500's and I don't like the fact it just suddenly cuts out when the battery gets low ... not great on a fast ish downhill on a bike with lots of ruts
If it's run time was as the rechargeable P5 claims to be (looked on their website for run time specs and supposedly 4 hrs min at 240 lumens with a 14500) at least if you have a reasonably charged battery before you go out you know it's not going to suddenly die as a car comes round the bend and doesn't see you or you hit a big rut you didn't see.
If I did bite the bullet and stick a 14500 in the M5 I wonder if I'd get similar run times or if blowing the boost driver may reduce it to much shorter times ie 40 mins in which case there's no upside.