A trade almost identical to this was proposed in a thread just a few days ago.
And my answer was yes. I would make this trade. Joe Johnson is a truely excellent player. I think his ideal worth is about 15 million per, but I think I'd be willing to go to 16. However, if he wants to go higher than that, i am no longer interested. He's good but not 18/year good.
From there, the hunt would be on for a big time big man - possibly even a borderline all star type like Chris Kaman or Al Jefferson, and a filled out roster of athletic guys who can run, play defense, and hit open shots.