Same engine =/= same requirements.
I depends on how well they optimize it. Texture quality, poly count, world density.
That is true, but it remains a good starting off point. Even if Zenimax completely retools the engine down from the skeleton, there will still be that engine's underlying weaknesses and strengths left over.
I'm not saying that TESO's requirements will be identical to that of TOR's, is my point, but they won't be radically different, either.