Sorry to hear this happen, here's a fairly comprehensive list of TSBs that might provide some aid, with built dates, symptoms, etc. 2012 Mustang TSB's and Recalls - LatemodelRestoration.com
Also found this on mustangrevolution forum: (regarding 2013 MT82 transmission changes) title: MT82 Tranny in 2013
They have made changes for the '13: Phosphate-coated gears ease break-in worries about gear facing wear noise. These go on top of the clutch and fluid spec changes incorporated in the 12s. One hopes they got a better handle on the QC of outsourced parts inside this thing as, imho, that's what's causing the early failures.
Just turned 3000 miles on my '13 and the MT82 is running fine. No car is going to be perfect--there are all kinds of compromises that must be made. The interwebz forums tend to amplify any issues. So far I have zero regrets buying my '13 GT (well, except, perhaps, that it took so long to get there).
I have an 11, and my buddy (who also had an 11) just got a 13 after being without a stang for about a year. We met up, did a short cruise, and then switched cars. Neither of us could believe that our two cars had the same transmission.
The 13 shifts so dramatically different that I thought I was going to stall it out since I couldn't feel the clutch engagement (not to mention it's a lot lighter).