Is there a way to force iTunes 11 to update tags?
From the MusicBrainz iTunes Guide:


  Updating Library in iTunes
  
  Brute force method (full library re-scan)
  
  This method works best if
    - you've re-tagged a whole lot of your files and don't remember which files/albums
    - or there are far too many for the first approach
    - or you've moved/renamed files and iTunes has lost them 
  
  Ready?
  
  
  Go to File > Add Folder to Library
  Point it at the top of your iTunes directory ("My Music\iTunes\iTunes Music\Music" on Windows, by default).
  iTunes will rescan your music, but recognize it's already imported, so you shouldn't end up with duplicate files (unless you have moved/renamed files) 
  


You can also refresh a selection of songs by following the AppleScript based solution (on the same page linked above):


  Mac-only advanced method
  
  This method is a bit more complicated to set up, but easy to use.
  
  Fire up Script Editor (which is under Applications/AppleScript, but it will work with TextEdit). Paste the following code, then save as a Script called /Library/iTunes/Scripts/Refresh.scpt (create the Scripts directory if it doesn't already exist).
  
  
    tell application "iTunes"
     refresh selection
    end tell
  
  
  Now, inside iTunes, you should see a Scripts menu (an icon of a scroll, just left of Help). The "Refresh" item on that menu will force it to re-read the tags for all tracks that you have selected.