Does more light come out the front? I don't care about the temperature, I care about how much light there is. If a change in temperature makes more light come out then it's worth looking at. If it doesn't, it's just irrelevant.
An aluminum Sinkpad claims to be 135 W/mK and copper Sinkpad is 385 W/mK, yet Djozz's test shows only something like 3% difference in output? That's within the margin of error! Shouldn't there be more difference than that, if it matters? Will the quality of the thermal paste have more impact than a 135 W/mK MCPCB vs. 385 W/mK MCPCB? Does a LED put out more light on a 20mm MCPCB than it does at the same current on a 16mm MCPCB? No! Because the area of a 16mm MCPCB is many times greater than what's needed to maintain a stable junction temp, and that shifts the bottleneck back to the spot we can't work on, between the die and the bottom of the substrate.
One interesting thing in his numbers, the Vf doesn't always correspond to the light output (and hence the inferred junction temperature). Higher temp equals lower Vf, so how can a run with the same fixed current but lower Vf (which means it's hotter!) put out more light?