German Taschenlampen-Forum.de has some tests of the 26650 Keeppower batteries mentioned in post #1. I’ve linked other reviews here so you don’t need to hunt them down.
Keeppower IMR 26650 5200mAh/20A und 4200mAh/50 !? | Taschenlampen Forum ← the 4200mAh 50A one (gist: pass on this one)
http://lygte-info.dk/review/batteries2012/Keeppower%20IMR26650%204200mAh%20(Black)%202014%20UK.html
Keeppower IMR 26650 5200mAh/20A und 4200mAh/50 !? | Taschenlampen Forum ← the 5200mAh 10A variant
Test of Keeppower IMR26650 5200mAh (Black) 2014
Test/Review of Keeppower 26650 5200mAh (Black) 2014