Efficiency wise:
That is an H2-C driving a 2S emitter with 1S battery input, sense resistor modified to ≈R018. Efficiency is ≈90+% and low mode is current spot on to obtain ≈700 lumens from an XHP50A. With an R020 low mode would be ≈590mA, a bit above 600 lumens and guaranteed more than 4 hours of runtime with a high capacity 21700.
All of these numbers are feasible and tested. 
