I *think* (correct me if I'm wrong) factory speeds for 150 grain bullets are:

.300 WM - 3290 fps

.300 WTBY - 3550 fps

.300 RUM - 3450 fps

The RUM seems to be seriously underloaded.

I took my new chrony to the range yesterday and was able to get 3500 fps from my Win. Mag. From the pressure signs, I wouldn't be comfortable using that load, but 3400 fps is very realistic.

Fastest handloads for the other two I've seen listed are:

.300 WTBY - 3650 fps

.300 RUM - 3874 fps (!)

As far as my limited experience on plains game is concerned, I think the more the better on the big stuff (eland, giraffe) (and 200 grain premium bullets) but for the rest, .300 WM is plenty.