I like him, he said "bum-end"..
300 something degrees? That's hotter than I thought..
I think the heat sync fins will work to some extent but as he said they are in the worst place, dead air behind the diff..
If you really wanted to you could run an oil cooler to it..
My car's diff is in the same case with the gearbox and at the Le Mons races they ran an oil cooler on them..
I bet you could design a diff cover pan with like a catch tray near to top to catch flying oil to put into your upper oil cooler line, and then have it go through the cooler and return to the bottom of the pan, and not need a pump..
My car also has heat sync fins like that on the bottom of the aluminum engine oil pan and it is reported that changing the underbody aero can be seen in oil temp despite it also having an engine oil cooler too..
So the theory does work, but the oilpan has much better airflow than the rear of the diff..
You might be able to just make a bunch of strips of steel about 2"X4" 1/8" thick and weld them like fins all around your diff case and get a better cooling effect than trying to do it on the cover.. It's all about surface area and airflow.. Add surface area where their is good airflow..
OR.. Genius idea.. (ha not really)
Put a little spoiler like air deflector over your diff like a shroud to catch the air from in front of/above it and direct it right over your pan with the heat sync on it..