If you are using the same powder in both cases 3-4 gr difference will produce comparable velocity. For example 64 gr of H4350 is listed as max for 180's in the WSM @2950 fps. 67 gr of the same powder in a 300 WM shows 2916 according to hodgdons website. So 3 gr less powder will give you about 35 more fps with the WSM.
But if you choose the FASTEST powder for each chambering there can be bigger differences. I've used some Alliant and their website shows 66 gr of RL 17 will produce 3082 fps with 180's in the WSM. The only 300 WM loads I can find will need 75-81 gr of various powders to get that kind of velocity.
Be aware, I found my rifle was showing pressure signs and was getting 3100 fps before I reached the max load of RL-17 and accuracy was not as good as H4350. I plan to experiment some more with RL-17, I like the speeds I was getting, but for now I'm shooting the slower, but more accurate H4350.