Dodds has Calvin outscoring the #2 WR by 28 points and the #4 WR by 35 points, which is about the same as the gap between the WR2-4 guys and the WR8-12 guys. Calvin is likely to outscore the other "first tier" dynasty WRs by as much as the other "first tier" dynasty WRs outscore the second-tier guys, at least in the short run. And in the long run... I'd feel much happier betting on Calvin Johnson, (one of the most ridiculous athletic outliers the league has ever seen and a sure-fire first-ballot Hall of Famer), playing well into his thirties than I would betting on Demaryius, Dez, Green, or Julio. In terms of expected games remaining, I might expect those guys to have a 20 game advantage over Calvin. But Calvin will outscore them all by so much on a per-game basis that yes, for me, he's a tier unto himself in dynasty.
At what point do you prefer someone younger? A 30 yo Calvin vs. a 27 yo Green?
I also expect Calvin to play well into his 30's but I expected the same from Fitzgerald.
Hard to say- it's going to depend a lot on how Calvin looks at 29 and Green looks at 26. Like I said, based on what we know today, I would bet on Calvin Johnson as more likely to still be excelling at age 34 than A.J. Green is. Part of that is because Calvin is a better talent. Part of that is because Calvin is already closer to the goal, so there's less time for something to derail him. So Green might have a 3-year advantage in chronological age, but that doesn't translate into a 3-year advantage in expected games remaining. It translates more to a 1.5 year advantage.
In standard scoring, over the last three seasons Calvin has averaged 15.5 fantasy points per game. The #2-6 receivers are exactly who you would expect (Green, Dez, Demaryius, Marshall, Julio), and they're very tightly packed, all finishing between 11.85 and 12.05 fantasy points per game. That's a 3.5 PPG drop, which is just a crushing advantage. It's basically the difference between that #2-6 pack and guys like Torrey Smith, Anquan Boldin, or T.Y. Hilton. That comparison isn't totally fair to the younger guys, but there's really not any way to split the data where Calvin isn't outproducing the second tier by as much as the second tier is outproducing the 3rd tier, if not more. If you look at individual seasons (minimum 8 games played), Calvin Johnson has three of the top five fantasy seasons over the last three years. Nobody else even has two of the top ten. Next best is Demaryius (4th and 13th best), Green (10th and 12th best), or Marshall (6th and 13th best). And that's despite the fact that
Calvin was historically unlucky over that span.
To me, the debate between Calvin Johnson and the next tier is similar to the debate between, say, Aaron Rodgers and Andrew Luck (or Drew Brees and Matt Ryan, or Jamaal Charles and Doug Martin). Yes, the younger guy is talented and should produce really strong fantasy numbers, and his career is going to be a lot longer... but I expect the older guy to just crush them in fantasy points. And since each point of VBD is more valuable than the point before, I would rather have three more years of #1 overall WR production than four more years of WR5 production.
Or, to put it another way... in 2013, according to PFR, Calvin scored 130 VBD*. In 2012, he scored 102 VBD. In 2011, he scored 149 VBD. That's an average of 127 per year. The #5 WR over that span averaged 79.3 VBD. Over the last three years, three years of Calvin Johnson has quite literally been worth 4 or 4.5 years of #5 overall WR production. Now, past results do not guarantee future production, and PFR's VBD baselines are a bit wonky (they use WR30 as the baseline instead of WR36, which does have an impact), but that's a pretty clear illustration of the concept, at least.
(*prorating his stats to 16 games)
Edit: Another point- let's say Peyton Manning, Matt Stafford, Matt Ryan, Tony Romo, and Andy Dalton all got hit by a bus tomorrow. Which of the "big 5" receivers do you think would be most impacted? Which would be least? Personally, I think Calvin would weather a backup QB a lot better than the other guys, which means Calvin is actually a less risky bet- there are fewer ancillary injury concerns. That's part of the reason I like Percy Harvin so much- if he could tear it up with Christian Ponder, he can tear it up with anyone.