The plug heat transfer thing is all about pre ignition, if you are getting pre-ignition or detonation (unlikely with stock) your temps will spike up, usually under high load. The electrode gets hot and either totally lights off the fuel or just makes it burn faster. That's what's kinda crazy about the whole heart range thing, it will actually speed up the rate that the fuel burns in the chamber in effect advancing your timing. In order for the fuel to burn it's got to absorb the latent heat of combustion so anytime you increase that heat you make it burn faster which has the same affect as advancing timing.
I haven't fucked with derbi much but when we were designing the derbi heads on the hobbit, we found those head shed heat really well. Seeing cht over 400 is pretty high for a derbi. But at the end of the day, cht really doesn't matter, look for scorching on the back side of your piston to know if you piston temps are actually too high. When we were messing with hobbit heads they could be 450+ cht and still happy clean piston just because the head sucks at getting rid of heat.
If everything else is perfectly tuned and you are just seriously making that much heat and power, you can switch to redline or maxima 927 or klotz beanol, those seem to have the highest temp stability. Anything with castor, and whatever crazy magic is happening with redline.