You have 15 cards but you're underclocking the memory?!
I belive 1080ti Works the same as any other from the 10 series, so I'd say play between 65%-80% power and depending on the algo try different overclocking until your system is stable and doesn't crash.
In most of the cases though, the core clock wouldn't make any difference (in Neoscrypt I set it to +50 on my 1070ti though, it gives a little boost at no extra power draw). The memory speed is the most important but you need to see what manufacturer you have. Samsung would be the best, it can be overclocked more than others. Oh and because of that reason, you'd better overclock every GPU independently. Some GPUs might be able to handle +700 on memory, others might crash after +600 (those are not 1080 specific numbers).