On spreadcoin I could only get a maximum of 9 Mh per GPU even with overclocks (100% TDP).
I get 9.3MH on the gigabyte gtx 1070 g1 gaming with oc on both the memory and coreclock.
f.ex core: +150, memory +500
On the 1080ti you can try to reduce the memory clock -500 and increase the coreclock. The highest reported hashrate from a user is 9.6 MHASH on the gtx 1070 and 16MHASH on the 1080ti