Why do you think that?
Radeon VI will be more efficient than a Vega 64, but it will use the better efficiency to push more pixels and polygons than the Vega 64, not to deliver the same performance with less power consumption:
I think perf/watt will massively be improved with the Radeon VII compaired to the RX Vega 64.
Look at what AMD did with their new Ryzen chips.
They beat out the Intel 9900k (which is 180watt+) and do so at 130watts.
They just had a massive improvement in perf/watt (for their cpu line), and at the same time finally managed to beat intel in cpu performance.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071