Windows 8 Pro x64 with updates and latest Intel iris Pro Haswell graphics driver as of 17 September (Intel Windows driver dated 11 Sep). Testing by Michael Larabel of MichaelLarabel.com for a future article on Phoronix.
Compare your own system(s) to this result file with the Phoronix Test Suite by running the command: phoronix-test-suite benchmark 1309198-SO-WINDOWSIR25