Got my XFX 7900XTX today. It seems that the drivers and the architecture are a little bit broken at first. Expect some NICE increases in the performance (Raster aswell as RT) and power usage.
Yep, AMD drivers upon release of a new product/architecture is usually a tad.....unoptimized, but I do expect performance and power usage to improve over time. Can't wait to mess around with my MERC 310 RX 7900 XTX when I get back!