Look, Stafford has played great, no doubt about it, but having watched several of his games there were many throws where he just went "I'm going to throw this as high as possible to a double covered Calvin" and he caught it.
Props to him for doing that, but it's still a great luxury to have the most physically talented WR in the game who also happens to be one of the fastest AND tallest.
What I am saying is that if you're a good Quarterback and you get an elite WR you can put up some great numbers.
A great example is Kurt Warner:
When he had less than Elite talent he was pedestrian (27 TDs to 30 INTs and an 82.1 QB Rating). When he had Elite talent he did amazing, in both St. Louis and Arizona.
Or Tom Brady:
Before he got elite offensive talent he had an 88 QB Rating and averaged 24 TDs and 13 INTs a year. Since he got elite Offensive Talent he's averaged: 107.3 QB Rating and had averages of 31 TDs and 7 INTs.
Or Donovan McNabb:
The first year he got an Elite Offensive weapon (TO) he threw a Career high in TDs (31 vs 25), Comp. % (64 to 61.5), and QB Rating (104.7 to 95.5).
So while you may have proven that Stafford would be a good Quarterback without Calvin Johnson, you still haven't shown that he would be a "throw 40+ TDs and 5,000 yards" QB.
But you know what, that's OK. Because Elite numbers like that take both a great QB and a great Receiving corps.
That being said, there are very few QBs in the NFL who could put up a 41-16 season IMO, even with Calvin Johnson. Many people say Cutler could do the same if given the same chance with the same receiving core. MAybe he could maybe he couldnt...but he hasn't done it yet so we will never know for sure.
I don't doubt at all that he could throw 40 TDs given the right offense, calvin, etc....but could he also limit his interceptions at the same time? that remains to be seen. to throw 40 TDs in a pass first offense you have to take a lot of risks and with risks come increased chances for interceptions. many like to point to his 7 int through 10 games last year and they have a point...he was very improved interception wise. But at the same time he was on pace for a rather pedestrian (by today's standards) 21 TD passes and career low 58% completions. Some of that is obviously his receiving core but maybe he wasn't taking the risks he had in years past leading to lower interceptions but at the same time lower TD and comp. % numbers as well.