Hi,
 
01] Does Windows 8.1 have some performance improvements over Win 7?
02] How would I setup my BIOS for the best CPU usage?
 
Regrds,
B

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).

No problems with Windows 8.1 Pro x64 here other than a lack of drivers for my Fantom XR.

I don't know what your BIOS is, but I have left mine at Default except for turning off number lock and some fan speed controls. 
 
My primary recording system is Windows 7 64 bit.  I recently sent the Gigabyte motherboard in for warranty repair, and I set up my backup system which has Windows 8.1 and "Start8" that restores all the Start Menu and desktop mode.  I installed Sonar X3 (and updates) and all my addons, such as Pod Farm 2 Platinum and Ozone 5.  I installed all my recording hardware and everything except the Frontier Tranzport installed in the native Win 8.1.  I had to install the Transport in a compatibility mode (Vista) but it works OK.  Everything works just like the Windows 7 system.  I see no difference in Sonar Operation, maybe even a boost?  as my main system has an i7 3770 CPU and the backup system has an i5 3550 CPU.,