If the 1080ti performs where you want it to—and nVidia hasn't disappointed in that area for a while—then it really doesn't matter what memory it has.
Also, wasn't there a rumour that TX Volta will be out next year? If that's true it doesn't matter if TX Pascal is ousted by the 1080ti because people could just wait another six months for big Volta. I don't think that'll happen, but it was rumoured.
I personally think the 1080ti, whether with GDDR5X or GDDR5, will perform below the Titan XP, but match it when AIB partners perform their standard overclocks and add their superior thermal and power control hardware.