I wouldn’t doubt if going to 2 bond wires vs. 3 did cause the Vf to go up, but I am more talking about what increased brightness as opposed to what increased Vf.
What can you see on top of the XM-L die that you can’t see on the XM-L2?
No, there isn’t. XP-G2 is a prime example. It has higher lumens per watt and a lower Vf than the original XP-G. Vf has correlations to current and to temperature.
So there is more than one way to take advantage of the increased efficiency. One is to use a boost buck driver and run the xml2 at 3A+. Another would be to run the xml2 at a lower current(2.6-2.7A) and realize the efficiency gain in run time keeping output more constant. Thus would allow continued use of 7135 based drivers without loss of time in regulation. Obvious stuff but might as well say it.