You might want to hold out until AMD gets the "Nested Page Tables" issue fixed.

It's supposed to speed up virtual memory handling in the guest by avoiding the need for shadow page tables and/or emulation by the host.

Currently the feature is broken, resulting in crippled GPU performance, allegedly due to the IOMMU not keeping up with the DMA transaction rate. You can disable it entirely (kernel command line amd-kvm.npt=0) at the cost of ~5x increased CPU usage and constant stuttering in games, which is what most people choose to do for now.