I'm struggling here too.
My Asus P8Z68-V Pro, with 3402 (modified) bios and i7-2600k at 4.8ghz boots 10.7.4 and I get ~ 17,000 in geekbench 64.
Same bios, board, setup, another partition running 10.8 GM - 11,000 in geekbench 64.
I have tried the speedstepper mod, generating dsdts and editing them. Various smbios.plists. Various SDST files. For a week I've been reading through the forums and attempting various fixes, I just can't get this mb to run at 4.8ghz in ML.
I have another MB - Asrock Z68 Extreme 4 that runs ML 10.8 at 4.8ghz no problems whatsoever. I don't even use a DSDT.aml.