I'm using an MSI GTX 980TI GAMING 6G, which comes with a beefy front-plate and VRM heatsinks pre-installed (these can be left on the card when installing the G10 bracket). The fan on the NZXT bracket + the MSI heatsinks seems to do the job nicely.
Did you install a shim between your gpu and H75? It is probably not making good enough contact if you haven't. The MSI card probably needs a shim like the classified does for example... because of the vrm heatplate that the card has versus the reference model.
