Right now, I think Vega 10's biggest problem that AMD didn't see coming (or failed to achieve with GF) is that the chip just hits a huge efficiency wall above 1.4GHz.
Vega 10 has 40% more transistors than Fiji while keeping the same number of execution units, halving the number of memory channels and bringing relatively few changes other than RPM. AMD stated that most of the additional transistors were put there to drive the clocks higher, but Vega 10 only clocks 40% higher than Fiji.
It's like the chip is gaining almost nothing from the 28nm -> 14FF transition, because 1.4GHz it's practically how much a GM200 can hit within 300W.
Then we have statements from @Nebuchadnezzar saying the 14FF process was never meant to be used with voltages anywhere near Vega's default values. And pre-launch statements from AMD officials saying Vega 64 would clock above 1700MHz (it never did, did it?).
It all points to AMD engineers planning on Vega 10 being able to clock comfortably at ~1650MHz with a ~220W TDP and 0.9-0.95V. And when the production units came up they realized the cards wouldn't clock above 1400Mhz at that vcore, so they panicked, overvolted and pushed the clocks as much as they could within a sub-300W TDP, which ended up being the 1-1.1V that we saw in the final product.
The 2-3% performance that Vega 10 loses in power saving mode makes it lose a whopping 30% in efficiency, or around 80W. And it was all just to achieve GTX1080 parity, which is a bit stupid IMO because driver tweaks usually amount to much more than that.
I think you mean gain a whopping 30% in efficiency.