If Capstone/Berger isn't willing to put this information in writing then they have not reached an agreement on a legally acceptable set of loads for this cartridge and bullet combination. Sure they want to help, they don't want to loose customers for all those expensive bullets, but they will not write it down (publish) so it can be held against them if something goes wrong. This is an indication that they have not reached the optimum loading range yet so you could be correct writing that the 300 WM data is close to that of the 300 PRC. It might not be later though...
There can be a couple of simple points with regard to advantages. The 300 PRC is a straight wall case without a belt. For those inclined to be 'belt haters' this is a sure positive.
Yet the 300 PRC also has an increase in case capacity using the dimensional increase for case capacity. (0.532" for the case head for the PRC as opposed to 0.532" for the belt and 0.512" for the case body for the WM) The case length and OAL loaded length is significantly different when you compare the SAAMI dimensions to the case capacity. Loaded with the same bullet (230 grain Berger) to SAAMI length we get:
77.2 grains of powder capacity for the 300 WM (3.340" OAL)
89.9 grains of powder capacity for the .300 PRC (3.700" OAL)
Increasing the WM OAL to 3.700" you get 83.9 grains, still less then the .300 PRC.
Loaded ammunition on the shelf though will go to the 300 WM simply due to longevity.