I took a moment to browse through a handful of the tagged questions here, and have some observations.
- Of the few questions I looked at, most did not relate strongly enough to SharePoint to indicate that they would be strictly better off on SP.SE, although the most recent one, as of this writing, certainly does.
- Some, like Sharepoint Sites on SP 2010, are directly SharePoint-related, and therefore would be fine to ask on SP.SE, but aren't necessarily not in scope for SU - I believe there is a range of questions for which the scopes overlap. In such cases, I'd suggest directing them to SP.SE only if they fail to generate any/enough useful answers here.
- SP.SE still has the lowest vote-to-post ratio in the network (possibly excepting any sites created after that post was written and therefore not included in the provided query). A side effect of this fact is that it can be very difficult for a new user to gain rep there, which may discourage users from posting more questions there after earning tumbleweed two or three times. SU has awarded close to 3 times as many Tumbleweed badges as it has Nice Answer badges; on SP.SE that ratio is closer to 200 to 1.
- A significant portion of SP.SE's questions are of a highly technical nature, involving aspects such as PowerShell scripting and server farm administration. I'm a Site Collection Administrator for a team site in a large corporate environment, and while I have access to do almost anything I want/need to do on that site, I have often found a question on SP.SE that exactly matches my situation, except that the OP had access to PowerShell and/or Central Administration in their environment and thus accepted an answer involving one or both of those utilities, neither of which are within my purview. While, before today, I had not considered asking SP-related questions here on SU, I am now contemplating whether I might actually be more likely to receive answers written for the user with the privileges I do have at my disposal by asking them here, instead.
- Thus, I would definitely not recommend updating the tag wiki with the proposed phrase as written - "Most Sharepoint questions would be better suited to sharepoint.stackexchange.com". However, I think that something more along the lines of "If your question is directly focused on SharePoint, consider asking it on sharepoint.stackexchange.com" might be appropriate.
All this being said, I'm hardly a top user on SP.SE, so take this with a grain of salt.