In general, the MacPro loses out in single-thread performance only, since Xeons are typically one architecture behind. When the Haswell based Xeons come out, Apple will refresh the Mac Pro line, and you'll get parity single-thread performance.
That being said, I wonder what the chances are that Apple will use the same socket for the Haswell-EP CPU daughterboard. Is there anything obvious that'd prevent them from doing that (e.g, chipset compatibility)?