I have W8.1 on a HDD for my 5,1 that I don't use very often but it runs great. I was running W8.1 on a 960GB Crucial M500 connected to an Apricorn Vel Duo x2 along with an M500 for Yosemite. That also worked very good.
I moved the SSD to my mini 2.3 quad and the same Windows install works equally well after a few changes (Windows key, Office 2010 key, PS CS6 key, computer name change, etc).