luxIII is 80 lumens at 750ma.
k2 is 100 lumens at 1000ma.
k2 is 130 lumens at 1500ma (1.5A!)
I'd be willing to bet if you can get a VF low enough that the output from a luxIII and k2 are probably similar at the same CURRENT level. Don't quote me on that though, it's just a guess. However K2's have a higher Vf in general.
EDIT:
Haha, i'm sitting here contemplating that, while i have the k2 datasheet open..Just not realizing it. :lolsign:
SO, link to
datasheet
k2 at 750ma eyeballing it looks like ~75-80% of 1000ma output, so something like 75-80 lumens?
Now, i don't know how the Vf will fall down from the rated @1000ma, but it will fall a bit. Overall, the luxIII is probably still more efficient though. Infact, i'm really pretty sure it is. I'll let an expert chime in on that one though