I don't know how the money would work out but I would cut Wilcox and sign Martin. Even though we have plenty of PFs (Bass, Sullinger, KG, Green), I think Martin would be a considerable upgrade over Wilcox and may make it possible to trade Bass down the road.
Of course if it puts us over the tax limit, I can understand why they wouldn't do it.
Kenyon Martin would be a much more valuable alternative at power forward than Wilcox. Kenyon would give Boston a true defensive presence at power forward (something they currently do not have). Someone who can take some pressure off of Garnett. Either when playing alongside KG or playing while KG rests.
Kenyon is also smart enough and tough enough defensively to function as a quick center when plodders like Darko and Collins are not an option + allows the team to keep a legit shot-blocker out there while playing small.
Kenyon Martin would be a really big improvement over Wilcox. A major reinforcement for this team's bench.
I don't think there's enough difference between Martin and anyone else to make it worth changing.
Martin is better at individual D and rebounding, Wilcox is better offensively these days and moves up and down better, and has familiarity on his side from last year. Both have health risks and are min contracts. Yeah, not enough reason to switch things up.
Collins provides the size, and that can be key. He is a keeper for that reason alone. Ditto for Darko.
Keeping Wilcox and Darko, Collins could be out, but only if someone is hurt. Basically, Bass or Wilcox would have to be injured long term AND Sully would have to not be ready at all AND Jeff Green would have to be forbidden to take any PF minutes to make it worth it.
I don't see Martin beating out all 4 of those guys; there would have to be injuries for Danny to make a roster change. If 2/3 of Wilcox, Sully, and Green go out long-term, then maybe there's room for Martin to have a role.