I like the idea of the Nano it's the one I'd buy, but I expect it will be too expensive and I have waited and waited for it, I fear the 970 itx could be a better alternative. The performance/watt of the Nano compared to the 290x could just be Amd marketing guff (similar to overclockers dream) if they compare the Nano to the 290x ref card using old drivers and high voltage+ throttling.
The x2 card looks to be dual Nano as it only has 2x 8-pin connectors.
Whenever AMD or NVidia or Intel make such graphs they always choose settings which makes their card looks the best or g shows the msot progress. They will test like 20-30 games and pick the 7-10 that give the best performance, and will spends ages tweaking and tweaking the settings. With the FuryX AMD performance slides there were lots of funny setting used, like turning AF off, 2XAA, medium settings.
The above AMD performance graphs are probably at 4K resolutions with some other oddities. That is fine, everyone plays that game. You just have to remember to take a bucket of salt with you when viewing their PR.