David Lee is probably the biggest Warriors' FA signing...ever, but that's not the point. Sure it is an improvement right now, we should have more wins than last year and we have a legitimate rebounding body right now, but it isn't the most logical fix.
Lee is many things, but a true center ain't one of 'em. The Knicks got away with playing Lee at the 5 because the East is still pretty weak overall. The West has teams with legitimately huge power forwards and centers: Bynum/Gasol, Cousins, Yao, TD, Nene, Kaman--I'll even include Sideshow Bob Lopez in Phoenix because he really showed me something last year, and he's definitely enormous.