Not just me either. Dale reported the same #'s on a U4 1C at about the same amps in the 1900's in a EE/BLF X6 no less 
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If you consider 1700's (maybe 1800's) was achieved with a U2 1A a while back at 6+ amps, then you bump up 2 bins, take off a little amps, it's all possible. Difference in output between 5.3A and 6.5A or so is not all that much - it doesn't scale linearly.
Edit: actually we got 1700's on a de-domed XM-L2 U2 1A in a TN31, post #128 here: https://budgetlightforum.com/t/-/17637. On a domed, probably in the 1900 area. I do believe though my amps to the LED measurements were probably low. Dale's method of using a quality loop meter is way better.