If the pieces are genuinely available, they will get to you. If you are not getting the final pieces due to hashfails, do NOT assume that turning off the ban on hashfail algorithm is at fault. Assume that it's either your connection setup or the peers you're downloading from. A peer that constantly sends you bad data will not stop sending bad data just because you're persistent. The more aggressive hashcheck algorithm will prevent you from wasting gigs upon gigs of transfer on bad data.