Hi there
RD580 is a lot faster than the RD480 boards with the ATI SB450 southbridge. However RD580 is only slightly quickers than the Asus and MSI boards equipped with the ULI southbridge. In fact I believe OcUK is exclusive Etailor on the MSI RD480 Neo board as ATI took all the other stock for their own systems. Also the pricing of the Asus and MSI Crossfire boards is extremely aggressive.
So RD580 boards will only offer a small performance boost over the ULI southbridge equipped motherboards (Asus & MSI) but RD580 does have a better feature array.