Your running dual 980Ti's...the only up from that is 1080Ti and it's really not that far Up, except in $$$...I could only dream of affording 980's much less 1080's...I'm an AMD guy (Hex Core-Radeon R-7)...I cut my teeth on marathon Render times of the day of single processor single threaded Rendering, I think maybe you're expecting too much from your Graphics Cards. Of course everything is Subjective but when it comes to High End Graphics Cards there really isn't a huge difference, some do some things better than others but do less in other areas it's half a dozen of one Six of another it's a personal preference. Personally 2.5 hours doesn't sound too bad depending on content of Render, of course if I could afford dual Nvidia 1080Ti's I might feel different...Patience my friend someone will come along with AI Graphics just think it up and it's Rendered....Poof!!

Remember...No matter where you go there you are