Some confusion developing here. I am just curious how things work with trucks, but the OP wants to know if he is "safe" with his set-up. He will get weighed and then more will be known about that.
It turns out that a DRW has stronger brakes than a SRW. Eg the bigger master cylinder mentioned above. Parts manuals could give more specs for the rest of the braking systems. I did find this too:
"With four back wheels instead of two, and a stronger rear axle, a dually truck can handle a larger amount of payload weight and offer more .....Most dually trucks are also equipped with bigger, stronger brakes for additional stopping power, as well as heavy-duty shocks and springs for greater capability to tread uneven terrain and rough driving surfaces"
I still don't see why braking specs have anything to do with axle weight ratings. I see that they would make the brakes stronger if the truck is expected to carry more weight. I would think they would use the GVWR for that. Whatever, the OP has a DRW so all he wants to know is how much "margin" does he have when overweight for braking while going down a mountain.
On "stopping distance", they have these roller things in brake garage floors that measure "braking" and perhaps that is how they decide how much braking a DRW needs based on the weight it is expected to be plus some margin.
I disagree with telling the OP he can ignore GVWR and just worry about RAWR, but nobody is going to change anybody's mind about that, so let's all just agree to disagree.
To me, it is all about the straw that broke the camel's back. The OP wants to know how many more straws will it take with his set-up. GM won't tell him!
On the question of using cab weight if you go over cargo weight rating, it is strange that the same exact cargo box and frame has different CWR if the truck is Diesel or gas. The Diesel engine weighs more so the truck has less payload out of its same GVWR. But the gas version has more CWR. So there they use front end weight to add to cargo weight. Using the little bit of cab weight (750 bs eg) for some extra cargo weight does not seem so bad then. So why do they even have a CWR instead of just a total payload rating?
Another confusion I started is about ABS. I thought it was different from "traction control". I should not have said "stomping" when I meant "brake hard". My question is whether you need to release or not to prevent overheating if you have ABS, and whether the ABS still does that (if it does) if you are overweight. I thought the OP would be interested in that too. He has had trucks for years so might already know how that works. I don't.
link to above quote
https://www.jdpower.com/cars/shopping-guides/what-is-a-dually-truckAlso "Wheel = tire and rim." Not to me. The "wheel" is the metal thing and the "rim" is part of that. The "tire" is the rubber thing that goes on the wheel. However, in casual talk, people often say "rim" when they mean "wheel" and "tire" when they mean the whole "tire assembly", so you have to figure out what they are really saying, which is usually not hard. :)