Favre had 18 TDs and 12 INTs when he got hurt. Let's not forget Kris Jenkins was a monster last year the first 11 games and he helped the defense be very dominant including six turnovers against the Cards that set up a short field all day for Favre to get 6 TDs. Look at some of Favre's numbers in some of the wins to that 8-3 record:
Cincy 189 yards, 1 TDs, 2 INTs, 1 fumble (won 26-14)
St Louis 167 yards, 1 TD, 0 INTs, 1 fumble (won 47-3 and T Jones had 149 yards and 3 TDs)
You put too much emphasis on Favre and not enough on the entire team. Their defense also fell apart down the stretch.
Personally, I feel they won several games early despite poor or average play by Favre. Teams win games, not QBs. Even Brady and Manning can't affect a team like you suggest. I argue with another QB the Jets could have easily had anywhere between 7-9 wins over that same 11 game stretch.