That's not too useful of a statistical analysis, as of course, a first rounder will be more successful than a later round player.
A more apt analysis would compare to success rates of becoming a 3 year starter in the specific round, i.e. comparing a qb from round 6 to position players success from round 6. A late round pick is rarely expected to be very successful as it is, so a 5-16% qb success rate might not be that low, relatively. As long as you're not relying on the player to be a starter/contributor right away, which nobody should be. Yes, it would be idiotic of a team to draft an expected first year starter in the later rounds.
It probably isn't that much greater of a risk to draft a qb at that stage, assuming you have adequate depth and ST/role players, which has not been the case for the Bills in a long time. And really, that gamble usually pays dividends as long as you can afford it (Like the Patsies/Packers with Cassel, Mallet, Flynn, etc). It did not work out for Levi Brown, but if the team trusts in Fitzy and we assume the team has sufficient depth (which I'm not sure it does), then a late round gamble on a project qb might be worthwhile.