I've not done a huge amount of additional fine tuning on the titan X (running the msi afterburner beta 10 auto OC "curve" setting), the 1080ti is my water cooled as per sig.
Tried a few benches but... firestrike for comparison.
https://www.3dmark.com/compare/fs/1...XQeKxNxiw96C803TvypZT4xvhAxn5K5S5juNJP-7go_8#
I know there's a decent chunk of difference on the physics score there... so... yeah, ok, that one example isn't the best but it seems to line up with stuff elsewhere too.
Just on the graphics scores, it's close or winning with a ~100mhz deficit.
Also (it's amazing how much validation one feels needs making when providing evidence

) that 1080ti clock wasn't 100% stable, pubg hated it. Much happier with the titan X.
If you look at CPU, the physics score is with lower CPU clock too.
I see enough issues there with driver versions/etc too. Take it as a general indication, not what I'm hanging my assumption on
it's just... odd. The titan X won't go much beyond that clock (1939 boost) without stability issues (will see, my waterblock is the titan X block so it might be on water soon). Most 1080ti's I've played with hit at least 2020mhz so I was expecting it to lose fairly badly.