Just as a data point:
The RGAWR on my 2021 3500 hd cclb srw gas truck is 7,250 lbs. The tires (stock Michelins on 18s) are rated at 3,640 lbs single at 80 psi, so the simple tire limited axle limit would be 7,280 lbs. My payload rating is 4,054 lbs, which I think is about 1,000 lbs more than a typical 2500hd.
My understanding from what MTU has posted is that, in addition to differences in the rear overload springs, the basic rear spring pack on the 3500 has one additional spring relative to the 2500.