Watts per square foot isn't the most reliable metric. In truth none are, especially what manufacturer's say about their lights. However when dealing with quality lights usually you want to shoot for at least 30 watts per square foot..if you start going much above 45 your plants will have a hard time without CO2. With lower quality lights, like cheap blurples or something you probably want a minimum of closer to 40 watts per sq ft.
HLG is a good company, and their recommendations I would say are probably their max flower recommended size.
Well as far as the heat goes, it all has to do with your extraction capacity and ambient room temperature more than anything. When I was running 420 watts in a 3x3 which is pretty damn intense, the plants liked it fine, but my 4 inch extraction fan wasn't enough. A 6 inch was though.
Another thing that a lot of people overlook is the lack of IR in most LED spectrums and if it is present to a much lesser degree than an HPS. So IR (infrared) light heats up the surface of leaves. Which is why when growing with HPS 74-77f may do you well in temp. With LED though, with the lack of IR the canopy or leaf temp is much closer to the ambient temp. So while under an HPS you may want 77 ambient to get an 84 degree leaf temperature, with LED your ambient temperature will be almost the same as your leaf temp. So you want to be running closer to 82-86f degrees.
