I think we have already seen what Matt Cassel would do with an average Team. For the first half of the season, he played behind a very Offensive Line, starting Billy Yates and the scrub pickup of the week on the Right side of the line.
The Team could only run to the left, and there was no pocket, as the right side of the line frequently collapsed. Were you ever impressed with Cassel running then, as he "stepped up" into the arms of a crashing DL? I thought not. With no time to throw, was it surprising that all he had time to do was dink and dunk? Then the inevitable criticism that he couldn't throw long. I remember the first pass he threw after taking over for Brady in the end-zone to Moss at about the opposition 40.
The Defense early, still had the 10 first year players to the Patriots that it does now, and its ratings were in the 20s in the League. The Offense ranked similarly.
With two thirds of the season gone and BB doing his usual superb development coaching job, those ten newcomers are "playing so badly" that the team Defense now is in the top dozen rankings.(actually 12th overall). I would suggest the players must be becoming somewhat introduced and familiar with each other. By now they probably even know each other's name!
This in spite of the poor records to begin the season. Cassel is a budding Star NFL QB, who has already played with an poor Team for half a year and done fairly well. Cassel 2008 is not equal to Brady 2007, no one is, but I think he has done as well as Brady circa 2001, if not better, has more athletic ability, a better body, more "escape-ability", and probably a better arm.
But remember those peripheral things count for little in my evaluation of QBs. What is between the ears and POISE are what makes QBs. Brady has developed and gone off the top of the charts there: but Cassel is already pretty high up there on those intangible but all important features, to go along along with his better athleticism.