The engine is essentially the same as for DXIW, but they have more time to get it running well.
I also heard a rumour that DXIW draws obstructed objects all the time, so that when you turn on your "can see through walls" augmentation you don't get a drop in frame rate. Sounds like a really bad move to me, and the rumour could certainly be entirely false!, but if it's true then we can at least be thankful that Garrett can't see through walls, and they can rid the Thief version of the engine of this highly unnecessary extra work.
Either way, I think the engine should work better for Thief than it did for DXIW.
Additionally, Ion Storm are VERY aware of the massive outcry from PC gamers over the DXIW "dumbing down", and will have the opportunity to not make the same mistakes twice.
Finally, as already mentioned, the Thief development team is not the same as the DXIW team, and they have several members of the original Looking Glass Thief developers on board.
Like a previous poster, my main concern (going by DXIW comments) is level size, but why don't we all just quit making *assuptions* that the game will suck, and wait and see what *actually* eventuates?