When you look at the "elite" QB's of the last 5-10 years, taking Eli Manning, Tom Brady, and Peyton Manning specifically...their career playoff passer ratings are as follows: 89.3, 89.1, 88.4.
Not exactly a lot of gap between them. Peyton's first 3 years in the playoffs were with teams that weren't exactly contenders, starting him off 0-3. Since Brady's sick run of starting 10-0 in his playoff career (including 3 titles), he's a more average 7-6.
I think there are a lot of factors that go into overall playoff records...and while the QB has a great amount of control on it, it's not always the end all be all stat. Some QB's don't get enough credit for their play even with a .500 or below record, and there's a few who get far more credit than they have deserved for a winning record in the playoffs as well.