I'm thinking those 3500 mAh 10A protected cells are the best choice out there now - the ones from MtnE have protection circuitry that has the least overhead/drain. They got more power than we will need for the stock setup, but I'm sure at the very least we will be doin some resistor modding 
.
If it's 4.3A at the LED, then the draw per cell would be in the neighborhood of 2.4-2.8A I'm guessing. There's some loss's from the buck driver on converting the voltage down from from the 2 cells in series (voltage adds). The most we can get out of the XP-L HI is limited of course, maybe 5A to 7A about for optimum output? Not sure - djozz has data on the XP-L (not HI) here: https://budgetlightforum.com/t/-/27500.