The opensource is slow. My miner is more than 20% faster on the 1080ti.
(the opensource speed of 760khs taken from the https://cryptomining-blog.com/12546-mine-vertcoin-vtc-after-the-switch-to-the-verthash-algorithm (with settings that does  58 MH/s for Ethereum))

My mod can be pushed to more than 900khash with more oc. The gtx 1080 does around 600khash