The thorfire vg15s is only running an XM-L2 at 2.8 amps.
you might as well go for capacity because no cell is really going to make any difference in the output of the light unless you go to a 4.35 volt cell like an E1 one or D1. If you have a charger capable of 4.35 volts then you should be running the D1 or E1 because it will keep it brighter longer than just about any cell out there.
I like the NiteFox UT20 over the thorfire. It’s slightly brighter, has onboard charging and dual switches with battery indicator light on the front one.
Just my $0.02