AMD are going to have a hard time even if they come close to Nvidia. But they don't have to beat Nvidia for a product to be compelling. Either way, assuming that Vega comes with HBM it could be very compelling (see Fiji vs Hawaii). Of course, I don't think that perf/watt is that important, especially at the high end (as long as it doesn't really sky rocket, obviously, and temps are OK). Raw perf, and obviously price/perf, is what's important.
Agreed, but power consumption will dictate performance characteristics, especially when there is a TDP ceiling that generally is adhered to (RX 460 vs. 1050 TI slot-only power and 300 watt high end single GPU's per the PCIe spec).
And by compelling, I mean new performance levels at new prices or existing high end at exciting new low end prices. The RX 480 did bring performance to lower prices, but it was unexciting because several last gen products were still faster, more efficient, and there were already several current gen products way, way faster and more efficient at already better perf/$ than last gen.
If Vega is 10% faster than the 1070 and is priced at $350, it's progress on the perf/$ scale but not very compelling or exciting. Also, I don't think small Vega will use HBM, but even if it does its just a bullet point to me. Mhz, die size, and memory types are great conversations but has zero effect on my purchasing decisions.