Yes, a healthy Matt Cassel at the end of 2008 is better than early 2009 Brady. However, a healthy 2007 Brady is better than everyone in the league. So what? Neither is an accurate or fair comparison.
Regardless of what Cassel did in 2008, that doesn't mean he'd replicate it in 2009. For one thing, it's not 2008. I know that sounds obvious, and yet, it's significant for several reasons.
2009 started with a new third receiver in Joey Galloway, and a rookie that was getting adjusted to the system in Edelman. Security blanket Wes Welker was injured and missed a few games, and still doesn't look 100%, which is significant considering Cassel is at his best with short, quick passes. Randy Moss had the back problems to start the year too.
I'm not suggesting Cassel would have been awful. I'm just saying the comparison isn't really reasonable. Would Matt Cassel have beaten the Jets defense? I don't think so. Would we have beaten the Broncos with Cassel? Doubtful with the way our D played, and Cassel is a bit shaky on the long ball so I wouldn't even say he'd have hit Moss on that bomb.