With the new XP-L HI it now has increased intensity but reduced lumens, I would it it is close to 1,000 led lumens but OTF is about 850 at best. The replacement driver was taken out of a Zebralight H600 1st gen and draws 2.72A, 100mA less than the stock driver.
1,200 lumens as claimed by the manufacturer is not possible to achieve with just 2.83A draw at the tail, but I would say it is very close to that, in LED lumens of course.
In terms of output the GH10 can easily stand against a flashlight rated ANSI ~1,000lm.