The current behaviour (placement of the new tab) if you "middle click" a link from tabs appears to be correct - seems to be the "Cmd+T" (or new tab button) functionality.
Interestingly, Firefox behaves the same as Orion does on Cmd+T - tab opens to the right of all tabs. The question here is if Safari's behaviour is "right" or "better" I guess.
If you consider tab history as a hierarchy with parent and siblings with order having a bearing (this could be cool for history navigation/re-discovery in future), then arguably placing child tabs next to the parent (including from Cmd+T), i.e. the Safari approach, might make more sense?