Its exactly the same for me, Crysis, Metro 2033, etc. are running about 10°C cooler than XIV.
The Nvidia driver will prevent 100% GPU load (and will also clock down if needed) for all known stress-testing tools, e.g. Furmark, OCCT, in order to protect the card. This rule goes only for the 580 so far and has been discussed on Nvidia forums recently after people were wondering why the older 480 beats the 580 in Furmarks benchmarking tool.
(Sorry for this being Off-Topic).
Square Enix is in charge to optimize their engine now....hope they will do. A patch for an optional DX11 or OpenGL 4.x mode would be great but i don't see that coming for a long time.