For example, Kerry Collins' 2002 post-season stands out in that he dramatically out-played his regular-season self. (It was only one game, but Collins was absolutely heroic in what was one of the most heart-breaking miscues in playoff history. With only a minute left Collins drove the Giants to the SF 23. A botched snap cost New York the game.)
On the other side of things was Tony Romo's 2006 loss to Seattle. Again, a bobbled snap cost the (anti-) hero the game. (Special teams plays are usually excluded in these kinds of analyses, but the play was technically a Romo run.)
Note that Joe Flacco's current playoff run ranks as the eighth biggest over-performing post-season (so far).
Hover over each point to reveal the QB-year. You might want to make use of the zoom controls.
Clicking on the WPA Rank tab shows a bar graph list of each QB's over/under-performance for each post-season. There is a similar analysis for EPA. One thing you'll notice is that the connection between regular and post-season EPA production is stronger. That's expected due to WPA's hypersensitivity to game situation.