My gut reaction was "we would have made the playoffs" but he wasn't the only problem down the stretch. Our defense had a couple shitty games. I think we would have needed 2 more wins to ensure a playoff spot (correct me if I'm wrong, in week 17 we needed to beat Miami and also have another game go right for us).
Our losses were Denver and Miami at home, SF and Seattle on the road. I think we'd still lose to Denver and Seattle with a healthy Favre, given the other elements involved in those losses. So he would have had to play well enough to beat SF on the road and Miami at home.
In the playoffs I think we had a shot against anyone else in the AFC and we spanked the Cardinals in the regular season that year. Could have been interesting.
I absolutely think we DESTROY Seattle with a healthy Favre, otherwise I largely agree with all of this. That Seahawks team stunk, and the defense actually played fairly well.
As far as Favre vs. Chad, you can't compare upside. I love Chad-he was what he was at that point in his career and that Ravens money was some of the easiest money I ever made in terms of placing the occasional wager. Who knows where we go with a healthy Favre.
I think the more interesting what if re: Favre is "what if he decided to not be a cunty little bitch boiiii who solely cared about revenge as opposed to a Lombardi, and stayed with the Jets for Rex's first year." Considering how lights out he was and how close the Jets came, that's a tough one to grapple with.