GW2 is not optimized for sli so your system is fine. Some games just don't use sli well and really nothing you can do about it on your end, even changing cpu will not change it. It was talked about in the gw2 forums upon release about the poor sli performance. In the future they may work with nvidia to improve this as they did add better triple monitor support a few months in. At first skyrim didn't support sli at all and great strides came later for support.