I'm no math wiz, but here's what I got out of staring at the Cree data sheets for a while:
MC-E:
2.8A x 3.4V = 9.52W
740LM at this drive level
740/9.52 = 77.7LM/W
XM-L:
3A x 3.4V = 10.05W
910LM at this drive level
910/10.05 = 90.54LM/W
That said, I still really like MC-Es. If you want to mod a light that has a Lux-V in it, an MC-E starts to look pretty good due to wiring flexibility. Also, if you directly reflow solder it to a chunk of copper...
the efficiency at higher current will be better than the data sheet might suggest.
The XM-L doesn't look like it would take very well to DR-C (direct re-flow to copper) as the package has no easy to reach leads.