Its not a bad deal, but I wouldn't do it--mainly because in your league offense is more important. You're losing a lot of offense to get a great pitcher but on a bad team (won't really help you in wins and losses-and based on the rest of your pitching I'm not sure Felix will be enough help to ERA/WHIP to get you some points). I would drop Ike Davis for one of the pitchers you named (I'm a big fan of Drew Smyly, he's been great. Keep an eye out for Trevor Bauer too, the D'Backs may be giving him the call sooner than later). I would then find someone who needs a 2B and try to trade weeks for a mid-range pitcher (someone like Colby Lewis or Cory Luebke or Vance Worley). The thing about trading Posey is you'll be left with a gaping hole in your offense at catcher (AJ is not the answer), plus you lose a lot of production from Hosmer.
i think you're underrating buster posey. i am partial to posey because i am a giants fan. he is an excellent hitter. more importantly, he is hitting to the opposite field. he hit two of his three hrs, all big shots, to right-center. he is in 2010 form right now. it's dangerous to assume any of the scrubs available in WW or FA will come close to posey's production.
the giants also don't have a completely worthless lineup this year, at least so far. melky is off to a good start and getting on base. pablo looks like he might be better than he was in 2009. the pablo-posey combo will be solid this year. posey will hit for average and power. that pretty much makes him one of the most valuable players at C. i wouldn't be surprised at all if he ends up being the number one C in fantasy. you will not come close to replacing posey with some WW or FA pickup.
i'm unsure if your team can lose posey's power. miggy is obviously a beast who will get 30+ HR. let's say hanley gets at least 25 HR. let's say luke scott gets you 25 or so (maybe optimistic). castro is arguably developing power but i would expect low double digit HR total. bourn will get a hr maybe. a jackson and murphy might get single digit hr. this is not a loaded team in terms of hr and categories linked to hr (rbi, ops, r, etc.).
also, trade hanley or castro for a top 1B. IMO, you're not using miggy and hanley as efficiently as you could. i think miggy is arguably the #1 1B and the #1 3B in fantasy. but he is the #1 3B by a larger margin than he is the #1 1B. same idea applies to hanley at 3B and SS. so trade one of your stud SS and get a top 1B. you control arguably two of the top 3 SS so you shouldn't have a hard time finding takers. i think this is a way to improve your offense's competitiveness without making any sacrifice.
i assume weeks will go to 2B when he's comes out of his current slump (he hit a 3 r blast today and flied out to deep right center). and i assume c young will replace a jackson in of or take the util spot.
so the final spot goes to ike, headley or hosmer. i am dismissing headley out of hand as i've heard his name pop up too many times in the past. i don't buy him in the medium to long term. i think hosmer is definitely the most talented player to fill your final spot. so i don't agree that hosmer is clearly expendable. ike might have a little more pop (i'm not even sure about that) but hosmer will put the ball in play and bat for a higher average and get those doubles.
your offense should be good enough to compete every week without tweaking it too much. strengthen your OF if you can.
i assume there is no innings cap in your league. there are a lot of talented enough pitchers on WW and FA. no one will be the complete package like king felix. but i think there are lot of cheap Ks out there. you can increase your chance of W (and L...) by starting/streaming as many pitchers as your rules allow. matchup plays can help out ERA, W, WHIP, etc. i think there's a way to game the pitching such that you will win enough of the pitching categories that you give yourself a good chance to win every week with your offense.
I went ahead and rejected the trade...I figured I can get by with matchuping pitchers and riding romero/shields. If i play matchups rights i would most likely be giving up K's an W's, but be strong in L/ERA/WHIP.
There is no innings limit so if need be i can always spot start guys.
C: Posey 1B: Cabrera 2B: Espinosa 3B: Headley SS: Castro OF: Bourn OF: A. Jackson OF: Trout UTIL: I. Davis BN: B. Revere DL: Plouffe
SP: King Felix, Fiers, Feldman, Minor, Shields, Dickey RP: Jansen, F. Francisco, Clippard, Cishek, Holland