2114mhz is the best it can do it seems cause of the heat. I even have a fan behind the end of the case to blow cool air threw the card slots that helps quite a bit but obviously don't look too great but does drop the temps like 5c to the first card with the card fans blowing 100%. Max temp no matter the clock is around 65-67c.
Evga's new pc case with the left side case fans would prob work perfectly to do this job with them blowing cool air in for the 1080s fans.
I'm prob gona go that route and order one. It's like that or a lot more to water cool the cards.