You know, it is possible, in fact even probable, that Castonzo is making a decision on what he feels is best for him regardless of who the QB is.
Does he want to prepare to play in the off season? Does his body feel right? Would his body feel ok with another couple years of playing the game? Does he want to delay getting started with the rest of his life? Those are the far more likely scenarios he is weighing and none of them involve who the damn QB is.
You can see separate instances where he escapes and delivers the ball downfield in a highlight video, but that's not the norm(like with other attributes it's not about absolutes - you can find film of most of those players doing almost anything (both good and bad)- the question is about the frequency and consistency of good vs bad, etc). Watch some full game cut ups(there are tons of them on YouTube) and you are going to see what I'm talking about. It's very visible with Fromm IMO.