I don't think we're overpaying Rasheed, although the 3rd year player-option does bug me. However, Rasheed was almost mulling retirement, so he could get us out of that contract and if he doesn't, he would make a good expiring in the future.
You need to remember however that this is very unlike Posey. First-off, big-men who do not require tremendous athleticism age well, Rasheed included. In fact, if you look at his stats, there has been very little drop-off, to the point that it seems like it was really due to slacking off due to being on a losing team.
Posey is also the opposite. A lot of his ability on defense and offense is through his athleticism. His defense is based on lateral quickness, and his shooting form stems a lot from his legs. That was one of the biggest worries, as once his legs give, so does his entire game.
I'm liking it, and getting very excited. It also means we would have the best defensive depth at the 5 position and never have to worry about Perk in foul trouble again.