With regards to efficiency, Cree website states that in cool white @ 350mA, the highest binned XP-C parts will produce a minimum of 100 lumens, in the Q4 bin.
They are currently only available to order in Q2 bin. A quick check at cutter, they have at best P4 binned emitters atm.
Compare this to the XP-G, which is readily available in the R5 flux bin (over 139 lumens at 350mA), and will produce probably twice as many useable lumens at the max drive level of the XP-C, and can be driven even harder when needed.
Given the smallpackage and die size, the loss of efficiency and limited drive levels are understandable, its not a bad package, but the cost of high performance LEDs is not the biggest factor for companies who are aiming for the "high end".
As Curt pointed out, he pays good money for the most efficient LED on the market, but its a small outlay compared to the R and D, machining/material costs and all the other things that make his lights as good as they are, so why compromise?
The XP-C has its uses as a low cost/compact static solution, especially for coloured applications, but when its going to be eating your batteries i'd rather spend an extra dollar or two on a more efficient solution.
Edit - tell those brass peaks to stop staring at me i want one lol