I live in NYC but I have central air running all day so might be why it's staying this cool. I have another 4 card rig of 1070's at my office and it's running about 73C. So far it's been pretty stable. I tried it at +2000 and it would crash each time. I have been running it like this for about 1 day without any crashes.
I did the Bios on the motherboard from a youtube video. Had zero problems running 6 cards. He claims you can run 12 plus but I'm sticking to 8. I replaced one of the M.2 with a pci-e card from amazon to get the extra riser connected.
Here is the video for Z270-A/AR bios mod.
https://www.youtube.com/watch?v=4cd1qoTAHmA