- an LED running at lower current is more efficient (higher lm/W), so they could give us a little more output at the same battery current, or lower battery current (and less heat) at the same output
- possibility to add an ultra-turbo mode for very short blasts. 3+ A to each of 5 LEDs... ooh la la
- a 5x XM-L might help to alleviate any donut hole in the beam (don't know for sure--just guessing)
Disadvantages:
- it'll cost more of course
- the head might need to be slightly bigger
- may need a more complex driver (e.g. can't use a simple on-off switcher like the Dry... unless you use 4x or 5x 18650!)
Right now they are having a hard enough time getting a decent triple X to work, so maybe once they perfect that we will see a 5 X come out. Both of my triple X lights the Skyray and the Dry are not even close to the operating efficiency of a single XML light.