There is a boost in performance, however you probably wouldn't utilize all the cores even if the OS allowed.
You'll mainly see improvements in video editing and such, not so much for gaming.
I run 8.1 and in my opinion if you were fresh installing an OS, get 8.1. If you already have 7 and everything works great, there's not much reason to upgrade.
It is true that Windows 8 will utilize the cores BETTER, not necessarily to their maximum potential. This mainly applies to the FX series.