Using www.pro-football-reference.com as a reference for statistics, you'll find the following data for QB's who have played in the NFL Playoffs. This comprehensive list only includes QB's where GameLogs were present. So guys like Otto Graham and Sid Luckman were left out just because of the nature of compiling their stats manually.
I surveyed a total of 58 QB's who started at least 3 games in the Playoffs. In alphabetical order you'll see a listing below with a summary of how they performed in the Playoffs compared with the regular season. Simply put a B is Better, an E is Even, and a W is Worse. I compared Completion %, Quarterback Rating, Y/A, Y/C and TD/INT ratio
27 out of 58 QB's did worse
18 out of 58 QB's did the same
13 out 58 QB's did better
Troy Aikman - B
Ken Anderson - B
Drew Bledsoe - W
Terry Bradshaw - B
Drew Brees - B
John Brodie - W
Mark Brunell - W
Kerry Collins - E
Randall Cunningham - W
Jay Cutler - E
Len Dawson - W
Jake Delhomme - E
Trent Dilfer - E
John Elway - E
Boomer Esiason - W
Brett Favre - E
Joe Ferguson - W
Vince Ferragamo - W
Joe Flacco - E
Dan Fouts - W
Rich Gannon - E
Bob Griese - W
Rex Grossman - E
Matt Hasselbeck - E
Ron Jaworski - W
Brad Johnson - W
Colin Kaepernick - B
Jim Kelly - W
Billy Kilmer - E
Daryle Larmonica - E
Eli Manning - B
Peyton Manning - W
Dan Marino - W
Jim McMahon - E
Donovan McNabb - W
Steve McNair - W
Joe Montana - E
Joe Namath - W
Dan Pastorini - B
Jake Plummer - E
Jim Plunkett - B
Philip Rivers - W
Aaron Rodgers - E
Tony Romo - W
Matt Ryan - W
Mark Rypien - W
Mark Sanchez - B
Phil Simms - E
Ken Stabler - B
Bart Starr - B
Roger Staubach - W
Fran Tarkenton - W
Joe Theismann - B
Johnny Unitas - W
Michael Vick - E
Kurt Warner - B
Doug Williams - W
Steve Young - W
As I said earlier, I am not trying to discounting Peyton's contribution to either the NFL, or his team [or any other HofF listed above ...] There is no denying the fact that most guys do not perform as well in the Playoffs [78% did the same or worse than their regular season performance]. This supports several of the arguments above. This also supports the fact that some actually do perform better, and they do deserve acknowledgement over their peers. Congratulations to the minority who rose to the occasion when the pressure was on!