The single-threaded PiFast test is used to calculate Pi to 10-million places, and it's a good indication of CPU performance. There's hardly any difference between Windows 7 and Windows 8 in this test; Intel's Core i7-3770K is equally fast in both.
Cinebench puts every available CPU core to work, and again, there's nothing in it. On a high-end platform, Windows 7 and Windows 8 both get the same amount of power out of Intel's premier third-generation chip.
PCMark 7 puts a cat among the pigeons, though. This benchmark examines overall system performance, and though most of PCMark 7's numerous tests are closely matched, Windows 8 has the edge in image manipulation (25Mpx/s compared to 13Mpx/s) and web browsing (183MB/s compared to 158MB/s), resulting in a higher overall score.