Although what you say about the case capacity of the RUM case is true, my own testing showed only a 20-30fps difference between it and the 375 Weatherby with factory ammo, I was surprised to say the least. We shot them side by side over the same chrony and the RUM was a LOT slower than I expected, even the owner of the rifle was totally surprised.
With my handloads and a max charge, I have been able to consistently achieve 2850fps with 300gr Woodleigh PP's, which is right up at listed RUM velocities in most loading manuals.
I think this is due to the fact that the Weatherby case seems to be quite efficient in powder to velocity ratio, which I noted at the get go when I first started loading for it, this seems to stem from the 1/2" of freebore, where the velocity rises quite well and pressure seems to be quite linear with no excessive pressure excursions.
I agree, the recoil is quite a step up from the standard H+H round, it's a lot sharper and gets your attention, but is not excessive, it's not as harsh as my 458Lott!