Compared to Dorian and ROnnie peak thats not impresive at all 
exactly.
Jay's back is still not that great.
eg here is Jay's back comapared to two famous backs, in similar poses:



Unlike many, I am not blinded by the hype. Just because Jay won does not mean his back has become fantastic.
Jay's back has gone from "not that great" to good- that is all.
It is not on a level of other "great backs" like Samir, Haney, Ronnie and Yates. (or Flex or Shawn for that matter)
If you compare shots of 2006 Jay to 2003 or 2004 Jay, his back does not look much different at all. It looks a bit thicker and wider, but thats it. It doesn't have more detail, which is what Jay has needed all along.