The WD tongue weight rating is for the DEAD WEIGHT of the trailer tongue, as if you set it directly on a scale.
Unless you are a college math professor, there is no way to accurately calculate the effect of the WD hitch. This is why dead weight is used.
15% of GVWR is a really good "worst case" value to plan for. In all but the most extreme corner cases (i.e. a heavy toy hauler with no toys), your actual tongue weight will be less. Normally it is around 13% of the actual weight of the trailer.
In all likelihood you will probably be just fine with your existing receiver.