I have the i7-990x and if you want to overclock that then you have to disable HyperThreading in the BIOS.
Just running it in idle with no overclock the difference in core temps is between three and four degrees Celsius lower with HT disabled.
Having said that I am a fish out of water on this section of the forum because I have not yet felt the need to overclock my system. I have no interest in doing it just for the sake of it. It is however nice to have that ability should I ever have my system succumb to a CPU bottleneck.
CoolerMaster HAF X, i7-990x, Gigabyte X58A-UD3R, 24GB Corsair XMS, Sapphire 7950 Vapor-X, Corsair Neutron 128GB, 3*Seagate HD (3TB), Seagate HD (1.5TB), Hitachi HD (2TB), Plextor DVD + BluRay, StorageWorks DAT 72, 29160 SCSI Adapter, Corsair H80