On average, how many passing TDs do you expect the top QB to exceed the 10th or 12th QB by? It's a lot more likely to be 10 than it is 20. That's a 20 point swing for QB, or a point per game.
Look at Joffer's numbers above. They are very telling IMHO and much more than the 20 points you are insinuating. I used those three years as extrem examples to illustrate the upside of having a top QB.
however, over that same 10 year period, the average difference between QB1 and QB10 isA - 25yd/pt, 6pts/TD, -2 per INT: 391 - 261 = 130 points
B - 20yd/pt, 4pts/TD, -1 per INT: 373 - 263 = 110 points
unfortunately those are the only two scoring systems for QBs that i have data for