The results was posted by Match in one of the new generation emitter thread tests.
The data/graps on vf was later removed by him. Might have been due to inaccuracies?
The data/graps can still be seen here.
If you look at Cree`s datasheets, vf on XM-L and XM-L2 you will see that it is the same. 3,33v @3A.
As long you use an aluminium pcb and are at 3,5A or lower XM-L2 will always be better than XM-L on a single cell on high as far as I can see. That is how I would guess most emitters are driven in single cell lights. If using better cells, you can push a bit further.
I think it would be better do just write.
“About 20% brighter than XM-L” on the XM-L2 comment.
(Tom E beat to to some of the info)