I don't think that will even be close to an apples to apples comparison. Zappe played the worst defense in the NFL, and with a bunch of backups. And Cleveland isn't exactly great shakes, nor is Chicago. Compared to Miami/Balt/Pitt, it's a different world.
Other than putting the Steelers (1-4 and lucky to have won even 1) in the different world from the Browns and Chicago, I understand what you're saying, but this is Fan Think, comparing how the teams ended up, like winning/losing, TDs vs INTs, etc etc, all the things we fans agonize over, or at least care about.
What coaches look at is how players did in the situation in which they were put. Did they make the right reads, right decisions. For us fans, throwing 5 pics in a game is horrible, but coaches look at things like, did the QB make the right decision, throw the ball to the right spot on the field.
I think they will learn a LOT from watching both QBs play 3 games apiece.
That said, I don't thnk there's a chance in hell that they go with Zappe. Just as soon as Mac can play, he will. And that said, I believe Jones and Zappe are both competing for the same job, and BB & Co will watch both closely.