I am sure it is overkill but why not. The majority of driving will be in the summer, so heat should not be a problem and when I am running it on the track
If you want thicker for overkill could pack it with grease like the @dukeboy_318 mentioned for the old powerwagons.
No?
That's my point.
Thicker is not better.
Better is the viscosity range that it splashes and sticks to the gears and maintains film strength and shear resistance while getting squeezed between the gears.
I do not know the heat generated in diffs while drag racing, but generally figure the factory selected grades after considerable testing.
Lets see if Direct Connection recommneded different.