I will agree that the 300 Win Mag is hard to beat all things considered but lets get a couple of things straight. The 30 Nosler has 4% more case capacity than the most voluminous 300WM case. What does that mean. While a fast 300WM may run with a slow 30 Nosler the average barrel from each will be separated by 75-100fps. That comes at the cost of 5-8 grains of extra powder and the barrel wear that comes with that. The 30-28 Nosler adds another 4% capacity the only thing with it is it will not push a 215 Berger to the next higher node. What it does afford is the same speed with less pressure and most likely longer brass life but it still requires slightly more powder. As far as what the 300 PRC will actually do compared to the others is purely speculation. Rich's PRCSI will most likely set right with the 30 Nosler when compared side by side. It is appealing because the lack of rebated rim will most likely hold pressure better than the Nosler case. As far as the RUM is concerned I have steered clear because every one I see running 215 Bergers is the same velocity or slower than the 30 Nosler. I think the RUM needs 230s to really shine. I just do not need that large of a bullet.
I don't have any first hand experience but here's what Wiki has for capacity.
300wm-93.8 grains of water.
30 nosler 89.8 grains of water.
28 nosler 93.8 grains of water.
This is why people are making the 30-28 nosler, to gain those 4 grains of powder.
Is the nosler getting real world speeds that much greater than the 300wm? I personally don't know but don't believe so.
https://en.wikipedia.org/wiki/Nosler_proprietary_cartridges
https://en.wikipedia.org/wiki/.300_Winchester_Magnum