McNabb has one career statistical area in which he leads all active QB's: TD to INT ratio. This is why he 'grades' as highly as he does. As for being overrated, I'll leave you to your opinion. I think he's solidly in the Top 5 among actives.
I happen to think most QB's are a little bit overrated when you come right down to it. I exclude Tom Brady from that last statement. I refuse to argue with Superbowl Rings. It's foolish to argue against the 'hardware'.
I saw your earlier listing of great QB's. Why'd you put Fran Tarkenton on that list? I remember him well, but the Vikings aside, he played for some pretty mediocre teams. I know he was career passing yards leader for a while, but he never really won anything.
The problem is that rings are a team effort. When it comes down to it it's really hard to determine how much a great QB does that is from his effort, and how much is due to a solid O-line and receivers with good hands. Brady is a superb QB, so is McNabb, but Vinatieri deserves as much credit for Brady having the rings as Brady does. And Bruschi. And someone from their line (it's sad when the most important part of a team are largely anonymous).