This presumes that the big ass heatsink metal can effectively receive the heat from the laser. This is part of why some lasers are pulsed: the average power is

This presumes that the big ass heatsink metal can effectively receive the heat from the laser. This is part of why some lasers are pulsed: the average power is not too high, but the instantaneous power is high enough to cause the surface of the material to ablate anyway. At which point your reflectivity goes to crap.