If this was an issue with the 1080 series I'd expect to see more people report the issue, the posts of very poor performance had one card in common which was 1080ti. I've seen two posts which suggest 1080ti can work ok, one of these posts was from Jura who appears to operate a 1080ti with two other cards in the same rig. My suggestion is to follow graytails post and tryout the same sample scene and compare render times to see if you have the approx same render time. note cross post with willdial.