I still think the 56 is the card to get in the 1070 to 1080 bracket, assuming you dont need the nvidia features.
It is cheaper (well it should be), it can be overclocked well, it can be undervolted [..]
It's between a 1070 and a 1070 Ti in both cost and performance. It can't be overclocked as well as either even if you set the fan speed on the Vega 56 to 100%, which is unusably loud. Even if you combine undervolting and overclocking and take the already very high power draw well over spec in order to optimise performance as far as possible, the performance increase is lower than that which you can get from overclocking even a 1070 with a reference cooler and with far less noise and a far lower power draw.
So it's not cheaper, it can't be overclocked as well and the undervolting you state as a feature is more of a requirement to prevent it throttling than a feature.
Here's a comparison review of Vega 56 and 1070, both with reference blowers, both with maximum overclocking and with the Vega 56 undervolted as much as possible since that's also required because it's far less efficient than a 1070. It's worth noting that the Vega 56 card failed to complete the full test suite because it couldn't sustain the overclocking and became unstable, but they used the results anyway despite it not being a comparable overclocking situation. So this review is biased in favour of Vega.
https://babeltechreviews.com/rx-vega-56-vs-gtx-1070-fe-overclocking-showdown/3/
Here's a more detailed look at scaling of performance and power consumption with both underclocking and overclocking, which shows Vega 56 scaling very badly indeed with clock speeds:
http://www.tomshardware.com/reviews/radeon-rx-vega-56,5202-22.html
The enormous increase in power consumption is definitely not an indication of "it can be overclocked well". Quite the opposite.
and is likely going to be far more future proof than the 1070 and 1080 as vega seems to be doing really well in dx12 and vulcan.
That's true, though.
The lack of aib models is a problem though.
You can pre-order an AIB Vega 56 model now. For £130 more than the same card with the reference blower. Which is useless - a watercooled Vega 64 would be a better Vega purchase for that sort of price.
I think that 1070, 1070 Ti and Vega 56 are all viable choices at quite similar prices.