For every bad starter who becomes a high end reliever a lot of bad starters just flame out of the majors. I don't think it's obvious that Sanchez goes from true talent 5 ERA starter to high-end reliever (true talent 3.2ish ERA?). There are a lot of warts there, and several of them would have to be ironed out for him to be a high end reliever.
That said, I agree with the premise of this post of yours: I've posted in the past about wanting to see what he looks like in 1-inning appearances, and have been wanting the team to try him in that role for a long time. I just don't think it's likely that the result is a high end reliever, and I certainly don't think that's his floor.