what are you using to measure the Heat Sink temperature?

I just clipped a little K type thermocouple probe onto the heatsink.   Not lab quality measurement, hi.  I did aim the little IR thermometer at it and while the reflectivity is too high for reliable readings the screw holding the FET to the heatsink seemed to be a hot spot and it tracked pretty well wtih the TC.

The typical resellers of these things have no idea what they are so take any claims with a few grains of salt!    As proof of that I offer this listing on Amazon for a much fancier supply with both HV and filament outputs:

When you see statements like " the general chassis of the gallbladder is a metal shell" - that's a clue to do your own homework ;-)