Right now, Peyton's 136.4 is the highest ranking a QB has ever had five weeks into the season, and second only to Tom Brady's 137.9 in week 7 of 2007 as the highest at any point after week four. This amazing start is unlikely to continue; we can expect his numbers to go down some from here. The question is, by how much?
Based on the passer rating leaderboard since 2000, Manning has already survived the steepest part of the curve. If his passer rating decays at the average rate for the league leading QB, you'd expect it to drop by a total of 7.6 points over the rest of the season. If that happens, he'll end the season with passer rating of 128.8, a value considerably north of Aaron Rogers' 122.46 in 2011.
In week 5, his rating was expected to go down by 3.0 points. He did throw his first interception, and it did go down, but only by 1.6 points. Of course, there's a wide standard deviation in these forecasts, but it'll be interesting to see how it plays out over the rest of the season.