It would actually be slightly less 'over bore' than the .257 WBY mag. The WSM case holds a couple grains less powder than does the WBY case.

Go over to shortmags.org and ask away there. Those guys are short mag freaks.

Here is a link to some .257wsm threads.
http://shortmags.org/shortmags/snitzforum/search.asp?mode=DoIt