He doesn't have much of a methodology and seems to have missed a key point of the heatsink: orientation.

If he'd mounted his Pis' GPIO parallel to the ground, heatsink fans perpendicular to it, he would have found his missing convection with or without a heatsink.

This is what I do, and mine haven't reported >45`F ambient under peak load in a passively ventilated area. There is no issue with heat removal with enough air space.