It all depends on the heatsink really. On regular aluminum ones, you can still run 4 amps, but only for a few minutes before you get heat sag and stuff. But there’s a guy who put an xml straight onto a copper heatsink and he did like 4.5 amps. Here it is He did over 6amps and after 5 minutes on that, the led only dropped 1367 to 1357 lumens. He could’ve ran it at higher amps, but that’s the highest his source would go. I’m sure you could take it to 9 amps.
The biggest enemy of leds, and just electronics in general is heat. Ever see those world records where people get some dry ice or liquid nitrogen to cool down the cpu, and overclock insanely?