With a bit of tweaking I've managed to get my Aorus Hynix 580's up to 838 H/s.  Can't seem to squeeze any more out of them. 
However with the same settings they can now achieve a stable 30.1 Mh/s dual mining ETH which is roughly 5 Mh/s more than I was getting from them to start with!

I just don't think the Gigabyte cards are that good.  They only have a single strap setting in the bios (0:xxxx) rather than the 1:xxxx and 2:xxxx other 580s have.  Not sure of this is a sign of their inferiority or not?

I've just ordered a couple of MSI Armor 580's which are supposed to perform better so will see what I can do with them when they arrive.