Charlie has a short article on what he thinks the die size of GK104 is:
http://semiaccurate.com/2012/02/06/how-big-is-the-keplergk104-die/
He did not, however, remove the heat spreader soooooo..............
While the stock gtx460 had performance about equal to a gtx285 at release (gtx460 eventually surpassed the gtx285 most of the time with driver revisions), most gtx460's were not stock. And, since the gtx460 was a cutdown version of the middle Fermi due to design issues Nvidia had, I think comparing BOTH the gtx460 and gtx560ti to the gtx285 is relevant. The gtx560ti was 50% faster than a gtx285, then I'm still expecting GK104 to be at least a little bit faster than the gtx580.