The line was a disaster last year. Despite the line, with Evans and Owens to throw to, coupled with Jackson's amazing abilities out of the backfield, those are some serious weapons for a QB to utilize.
We had a few close games (five losses by a TD or less) so I can't help but believe that we would've won those if we had a pro bowl QB like you suggest. Therefore we would've gone 11-5 and made the playoffs.
What a difference a legit QB could make!