Ted, why didn't you just replace the Nano? Sounds like it worked really well for you.
Darth, I tried it as a thruster with FCS Performers in medium size. It sort of wiggled around and it lacked drive in really good waves (4-5 walls and barrels) so I immediately thought it was underfinned. I'm a pretty big guy - 6'2" and 91kgs - so I have had that problem occassionally before (an Al Bean comes to mind). That was in the morning. Straight back to Yahoo and they swapped the fins for the same fin in large style. In the afternoon I surfed some dodgy fat burgers (3-4') but the board felt much better, but a little slow out on the flats. The next morning I went to a really fast ledging barrel and the board went way better. Drove down the line, handled the tube really well, and, when I didn't go looking for a barrel, it turned nicely in the pocket. I felt it still wasn't quite dialed in and two weeks later I got the rear fins and finally the board was nailed. Bare in mind I am not a big fan of quad set ups so kudos to me for sticking at it and being flexible.

Now I think the shape is amazingly versatile (Yahoo Wahoo). I have surfed it in thigh to chest high Bears and loved it and 4-6foot Dolphins, Tombies and Centers and loved it. So it's a very flexible little beasty. In the solid waves it was amazingly fast with really long carving turns, yet tight and whippy in the pocket when I was doing my tube stall (that's the board I was on when I got bounced and dragged across and urchin infested nigger head at Dolphins). At tiny Bears I was doing all sort of funky small waves slides I thought had passed me by.
It was the board I bought instead of a Nano, Vader or Vanguard in the middle of the year. I couldn't get one in the size I wanted and there were no test boards available anywhere near those sizes anyway.
So, like lacey said above, I knew from the outset it was fins and not the board, so that's a bit different from jumping on a board and going, "Oh god no!" I have done that a lot in single concave boards.