Thanks for the stats and the legwork. The stats don't take into account though the organizations that surround the players taken or who was already the QB on the roster when taken. Sometimes, players just luck into better situations that allow them to play and blossom than others.
If we were really going to compare, we should look at what the success rate is for other position players. Although I suspect the success rate would probably be higher, it would be interesting to compare. I would guess again it really comes down to whom the scouts think will succceed, the desire and skill of the player, and the competition on the roster. If a scout has identified that a QB is the best player available for the team based on who is available, by all means go for it. I would rather roll the dice and win 5% of the time, than never win at all by not taking a chance.