Isn't the Fury Nano something like 1.5x the Perf/Watt of the Fury X? On the exact same architecture?
Plus Vega will presumably have the advantage of using HBM over Polaris.
For reference using the AMD slide on Polaris, Vega, and Navi roadmap. Polaris is 2.5x the base GPU. Vega is very slightly more than 4.5x the base GPU and very slightly more than 1.8x Polaris. And while Vega will support HBM unlike Polaris, there is no indication whether all variants of Vega will feature HBM. If even one version uses GDDR (similar to 1080/1070 using different memory types) then it'd be questionable use to HBM to tout the power efficiency of the GPU architecture.
Remember that slide was about the architecture and not the graphics card implementation (unlike Fury versus 390 which was a card comparison and not an architecture comparison).
Fury Nano was also significantly more power efficient than Fury X and that certainly wasn't due to HBM but careful binning as well as running it in a lower power state.
Edit - think of it another way. When Sony and Microsoft were planning out PS4 Neo and XBO Scorpio there were no Polaris or Vega chips. There was only a roadmap. Assuming AMD presented Sony and Microsoft with that, they would know that AMD's projections were for 2.5x perf/watt for Polaris in 2016 and 4.5x perf/watt for Vega in 2017.
Sony goes hey, that's fantastic we can have 4.4 TFLOPs (4.5 TFLOPs for the same perf at the same power consumption using 2.5x for Polaris) in roughly the same power envelope as the PS4 and we can potentially have it out by the end of 2016.
Microsoft goes, hey, that's fantastic, we can have a 6 TFLOPs (5.85 TFLOPs for same perf at same power consumption as XBO if using 4.5x for Vega) machine in a slightly higher power envelope and we can potentially have it out by the end of 2017.
It could all just be a coincidence. Hence I'm not saying with any absolute certainty that this is the case.
Only that it would appear that it's more likely that XBO-T is using Vega versus Polaris.
Hell, it's entirely possible that Vega is only slightly different from Polaris, in which case we'll potentially have the Xbox Scorpio at 180-200 watts (higher than X360 launch edition). Ouch! I don't think Microsoft wants to go there, but who knows.
Regards,
SB