I was under the impression that part of what made bittorent work was that the protocol tried to estimate how much each peer is uploading moment to moment and only provide it that much data to download.

Oh definitely not, a limit that harsh would prevent most people from getting the whole file.

Uploaders get priority. But if you show up to a torrent past the initial ramp of growth there will be plenty of bandwidth to go around and you'll experience a high speed download regardless of your ratio.