Patrick, we have a similar issue with the Spool not assigning correct priorities:
http://portal.smartertools.com/community/a87198/spool-priority.aspx
But we couldn't duplicate your issue (tried both SM v14.5 and v15). It correctly assigned a different priority to messages sent by the user specified in the override.
On a side note, thanks for bringing this feature to our attention. Been using SM for a while and never knew this feature existed. Like you said, good for users who send newsletters.
Suggestion to SM -- make this feature more visible by moving it from Routing to somewhere under User Management. Seems like it would fit nicely on the Throttling tab when editing a user.