Lots of good info in that thread gords posted above including discussion on S2 vs. S3. I also linked Match's thread on XM-L (& other emitters) testing (output at various current levels) in post #6 in that thread - here is link to Match's test (note that a T6 bin was tested, so U2 would be slightly higher). At 1050mA you're looking at +/-395 lumens, vs. +/-852 lumens at 2,800mAh according to that chart. (Bookmark that thread too - I reference it all the time.)
-Garry