I just shot some 300 WM loads over the chronograph, and saw no difference in velocity between reg and mag primers. However, it is summer. This past week I tracked down a carton of the mag WLR primers as I think a difference in performance might show up in cold weather. Why not give myself a little edge if there is one to be had?
Read a Barnesness article about the 270, wherein he showed better & more consistent performance when using mag primers.