I could downvolt eth only to use ~80w stock clocks, GPU-Z (not at wall, but same card so it's relevant). ZEC uses similar. If you downclock to say 1000MHz instead of 1260, you get same speed on Eth (zec suffers), but you can lower voltage another 100mv or more, chopping off perhaps another 20w (haven't tested yet).
Sure you can downvolt to that extreme but your hash per watt suffers past certain points.
Anyways, good luck doing that 2 times a day per card if you're using the RX series if you're wanting to mine whatever is the most profitable.
Sapphire 470 8GB (1750 strap)
Zec 1260/2000 -i 2 1020mv, 82w GPU-Z Avg, 177H/s
Eth 1075/2000 820mv, 58w GPU-Z Avg 27.5MH/s
You can control clocks and voltages with Claymore's miner easy. Recently Zec and Eth similar profit on Polaris, and Zec is a lot better on 7000/200/300/Fury, so switching on 470 makes no sense.