I didn't realize this difference until just now. That is nice they provided a 1/4 20 mount and milling in the tail for a sling mount.I would hold off as well. To my eyes outside they look identical. The ff5gt has a bright Corona which ends abruptly to the spill. The ff5 has a narrower Corona and a secondary faiding Corona that transitions into the spill. The hot spot looks the same intensity on both. Need a meter to show the output difference. So it basically looks the same and gets hotter much much quicker. If you have an FF5 this light is definitely not worth it.
The one thing I do like about the gt is the ability to sling mount it. Might keep it for this alone.
I think they would need a bigger host and bulb to get 15k lumens. This ones just not cutting it. The other thing I failed to mention the FF5 when it warms up it reaches max brightness and does not dim.I didn't realize this difference until just now. That is nice they provided a 1/4 20 mount and milling in the tail for a sling mount.
I'm still really curious about the output situation though. It just seems odd that they would miss the advertised output by 4000 lumens, when the normal version pretty much hit the mark. Also the fact you mentioned that your FF5GT heats up faster than your FF5 says that the ballast isn't translating that extra power to the bulb but rather just generating more heat. So maybe 150w is correct but it's just way less efficient at that level than at 100w. Which is really unfortunate. That's the only thing that's keeping me from jumping on this.
Left is GT. not a huge differenceWhich is which? And again one is pointed higher than the other skewing perception. I don't see any real difference other than one looks warmer than the other. The beam looks cleaner on the one on the right and he has it shooting through grass for some reason. They both look good though.