The best and cheapest mod there is: the Mdocod 2X 18650 battery holder! Will work with MN15 and MN20 perfectly. This battery holder is really affordable, and 2 or 4 of those 18650s are available anywhere too. Finally, guilt free Lumens for your M6. Now you can REALLY start to enjoy your M6 without constantly thinking: "oh, no, those batteries are expensive!!"
As already mentioned before, Lumens Factory makes some GREAT bulbs also, the HO-M3T being my absolute favorite bulb for the M6 combined with 2X 18650. It draws 2.0 Amps if I remember correctly, and a decent set of 18650s gives you a full hour of runtime, without drawing them completely (this is VERY bad for any Lithium ion cell).
It's bright, has tons of throw (more than an MN20!) and not too much current draw. The perfect blend!