The WDH redistributes weight from the rear axle to the TT and the front axle. In effect, the weight is still there, it is just spread more evenly. If you weighed the axles, you would find that the front takes on more weight as well as the TT axles, so yes, one might argue that you reduced some of the weight on the tongue when you sent it to the TT axles, but you are still hooked up to the same trailer and it still weighs the same as it did before.
Payloads have been discussed to death, but what it comes down to is the weakest link. You could have a 1000hp deisel engine, but mount it to a weak chassis or couple it to a lightweight rear axle and you still have to stay within the limit of the weakest component. The trucks are now tested by the SAE and are proven to their standards of what they are capable of.
TV: '06 Ram 1500 SLT QCSB Hemi 4x4
TT: '12 Grey Wolf 27BHKS
Tow Acc: Tekonsha P3 BC, Curt round bar WDH w/sway control, CIPA custom fit tow mirrors