This type of grading system is probably arbitrary and relative. The grade is likely skewed by looking at not only Patriot teams of the past but also dominant teams of the past. The measuring stick for what grade the 2011 Patriots have earned is not relevant to those past factors. The measuring stick for 2011 should be entirely based on what the team's play is this year compared to the play of other AFC teams this year. What the NFC has done is also not relevant because the only measuring stick that matters after 6 games is progress towards getting the #1 seed. It is and should be the exclusive factor for judgement. Based on the Patriots being 5-1 and would be the #1 seed in the AFC, the Patriots have earned an A+. What the other stats say is that whatever the Patriots haven't done well, other teams have done those and/or other things less well.
Stepping back one level and looking at how their offensive, defensive and ST play stacks up against other teams, this reveals elements that are, obviously, not A+. It also provides fodder for what may happen. But it can't be used for the 'week 6 grade'.
For fodder's sake, grading the offense and defense based on points scored and points allowed is easy. The offense gets an A+, the defense gets a D+/C-. ST probably gets a B (based upon FG attempts that are successful, net punt yardage, net kick return yardage, ST scores allowed)