Let's start with the mother of all caveats...I'm not particularly high on any of them. Now that we've got that bit of obviousness out of the way, let me applaud Sean for the interesting question.
Plaxico -- I'm not surprised he got a deal simply because there's been talk of him getting a deal, and lots of "on the record" suitors. Ultimately the Jets are taking a flier on him, and I'm not sure Plax, even at this point, isn't going to give you most of what Braylon Edwards would give you. I think today's reports that Cotchery has been given permission to seek a trade tell you that the Jets really believe Burress can be a reasonable second fiddle to an emergent Santonio Holmes. That said, if the Jets have their way it'll be winning games with great defense and a stout run game. And Sanchez may be a "winner", but he's still a terrible quarterback until proven otherwise. So with all that in mind, I'll say he has 60 catch upside, but could be in the 40-50 range. He's probably still got red zone applicability, so I'll say 4-6 scores.
MSW -- Any time a team lets a 26-year old starter go, at a position of clear need, it should be a warning sign. Putting that aside, Sims-Walker has scored 14 TDs in 28 starts, for a team that's been anemic producing TD passes otherwise. He's been reasonably consistent on a per catch basis (in the 13+ yard per catch) over his three seasons, and as MOP noted, he appears to be in line for a starting spot on a team that will have Josh McDaniels calling plays, and Sam Bradford slinging the ball. I'm not sold on Bradford (as a real-life NFL QB) as much as others are yet, but he threw 590 times as a rookie and the Rams have done nothing to make me thinking they can/will run the ball more in 2011. So MSW COULD be in line for quite a few targets. The tough thing though is that even without re-signing Clayton, the Rams have tons of possible alternatives and it wouldn't surprise me at all if Sims-Walker was a marginal contributor by midseason. He's one of the hardest "starters" to project IMHO, and I wouldn't feel comfortable paying more than a later round pick on him hoping that his upside case pans out.
Roy Williams -- On one hand, I look at Roy Williams inability to star with an ultra-accurate QB at the helm like Tony Romo (who is also quite able to throw the ball downfield mind you), and wonder if Jay Cutler is really the tonic that ails him. On the other hand, Martz generally gets production out of "his guys" and I suspect the Bears will throw more than they did last year for a variety of reasons. I like Williams in best ball/survivor type formats, probably more than the other two, but in this offense I still think there's a fair chance he catches 50 or less receptions. I still view Knox as the #1, Bennett is a factor, and while I think Hester should play special teams exclusively, you can count on him in Martz' famous 4-WR sets.
Net-net, Sims-Walker is the guy that wouldn't surprise me to see a Top 20 season, but he's also the guy I could see totally flaming out more than the others. I think Roy Williams is probably the surest bet to be at least a WR3 in deeper leagues on most weeks. My projections for Burress are aggressive, and to be honest unless I see evidence in practice quickly that he looks like his old self, I'm going to have to ratchet them back.
