The amount of weight "taken off" by a WD hitch is variable. There are no fixed amounts or percentages you can count on. You can't use one to overload your truck by XXXlbs, thinking "Oh, the WD hitch will transfer that to the front axle and the trailer."
By the way, receivers are rated by the "dead weight" of the trailer tongue in both weight-carrying and weight-distributing mode, not the weight after the WD hitch is applied. You can't hang a 1300lb tongue on a 1000lb-rated receiver and say, "Oh, the WD hitch will transfer that to the front axle and the trailer."