I have Gigabyte G1 Gaming GTX1060's with 6GB Memory (Samsung and Micron memory mixed) They all do well at around 23-24MH/s
I also bought a Gigabyte G1 Gaming GTX1060 3GB with Hynix.
It gets only 18MH/s with memory clock of 450Mhz. Anything higher and it crashes (The 6G cards are running at 650 or 750 mem clocks)
Nothing you can do about the Memory performance though. Silicon Lottery!
Memory clocks are important to Dagger-hashimoto Algo so yeah, agree you might be better off mining ZEC which favours the GPU Core more.
You could also Dual mine Eth and maybe Sia. I see it as last chance to get Sia coin now as June 2018 the Asics miner will be released for Sia Coin.
https://obelisk.tech/At least Sia Coin will serve a purpose for the future and HP is certainly interested in it.
https://minebox.io/