I have one of those Asus ITX 970s, it does not have heat issues, overclocks well and performs similar to a full size GTX 780 with a triple fan cooler
A R9 390 can beat this card however, so a 390X would beat it by far, so if the Fury Nano can beat a 390X then it would beat my card by a significant amount.
However, a Fury X has been shown to fit inside mini itx cases and perform well in that situation.
And that's the point, the FuryX isn't exactly a large card, it just has the radiator to house somewhere. The nano is going to need more than just the small size to make it sell well, performance at HTPC resolutions will be crucial.
The whole Fiji range are good chips, seeming let down by outside influences. Price, marketing, availability, connectivity ( I think the HDMI 2.0 might actually be internal to the chip).