I wasn't really thinking of 'perfecting' anything, more about playing reasonably fluently, with some dynamics and pedalling, at a reasonably even tempo. When the OP said he moved on even when playing pieces 'poorly' i had in mind something well below this level.
I find that if i concentrate on one aspect of a piece, often everything else (tempo, dynamics, and pedaling) can go out the window. So for me at least, learning a new bit of technique involves some learning in isolation, following by integrating that skill with the others so that i'm doing everything at the same time.
Does anyone return to method books later on for sight reading practice?
I had thought that practising sight reading would mainly involve having a large volume of pieces to practise with, and that method books would be good for this.