Thanks for your valued for reply,
I have stock settings for all, same series and model cards MSI NVDIA GEFORSE GTX 1060 3GB
All have tmp around 68 and fan speed 60%
Definitely sounds like the 15Mh cards are hynix memory. You can stil OC them but they wont do aswell as the Micron or Samsung memory type.
Get MSI afterburner and tweak the settings. ETH is all about the memory clock really.