I think the difference would be minor (a few mm²?). One of the key benefits of NVIO isn't die-size reduction (I think), but yields and manufacturing price. Defect in I/O section would kill entire 500mm² GPU. SPs, TMUs, ROPs and MC can be switched of and GPU can be sold as a cheaper part, but I/O abilities are essential.
I'm curious, if the yields and price benefits of the NVIO are still present during current aggressive price war - e.g. it can be good to invest a few more $ to rescue some GPUs, which can be sold for price X, but is it still profitable, if you must sell these GPUs for price X/2 or X/3?