In the XM-L2 datasheet you can see performance loss uptowards 150degree celcius. Id guess 150 is good then. J)
As far as I know. XP-G2s and XM-L2s can die if they become too hot and desolder off from the mcpcb. MT-G2, just put it back on and let it rip. Seems quite indestructible.
The hottest ive tested a light is 140C/280F on the head. Modded 9 XM-L light. Used welding gloves. Changed batteries in that run in order to keep the temperature on those below 60 degree Celsius. Batteries should be kept below 60.