I had a very bad experience with 8.1, but Win 8 works really well (especially under Classic Shell). From the little I've read, Win8 is a more improved version of Win7......most people just don't like the Metro/tile looks.
Do you have Win8 already installed as 8.1 is/was a free upgrade? If you have already purchased Win8, you can set up a dual boot config. and test out Win8 to see how it fares out. Before doing that though, you should verify that all your hardware (audio interface etc...) have solid drivers that support Win8.
As to the BIOS question, depending on your MB and it's BIOS utility, you'll have to check that and see for yourself (generally, the latest BIOS version should already be set up for best CPU useage.....at least it was on my board).