Oh, and I took this out of the previous post (too long) cause it wasn't wholly relevant, but I still think it's rather interesting information (though I have no solid proof since I don't have a bunch of advanced equipment as well as Sampras' rackets and so on). It is purely theoretical (most of it), but I feel it's quite logical and it explains a lot of what is already confirmed and what I read on these boards (which isn't always true as we know when it comes to conspiracy theories of pro rackets).
The ProStaff Tour, which the n90 design is based off of, was meant for Pete Sampras. Sampras would place a lot weight around that general area where a majority of the ProStaff Tour's weight is centered as counterbalance for the lead he puts on the head (or at least Nate Ferguson did it). With this, Ferguson would have to put less lead in his racket and could just slap it on the head and be done with it. (Yes, Wilson will occasionally make rackets specifically designed for top pros and sell them to the public, which is a mistake since those rackets were incomplete and require lead to finish it.) This is another reason why the K90 is so popular - it's a finished product. Federer has been using that EXACT racket (specs, weight distribution, EVERYTHING) for 7 years now. Recently he seems to be adding a little bit more weight to it, but it's hardly a noticeable difference aside from more comfort (it could explain the recent pile up of errors though in the past few years; yes I said it wasn't a noticeable difference, but some people can feel the performance difference and I'm sure Federer is one of them). He probably did it in his attempt to create a heavier ball and win the French Open. Everything he's done in the past several years has all been for that. If he never cared, he'd still be ripping relatively flat, penetrating winners all over the place. Now he hits with far more spin and might have changed his racket setup slightly since 2006 to help that. All I really know though, is since his switch to the K90 (or him staying with the K90 but with the K90 paintjob, whatever you prefer), is that his groundstrokes are occasionally very erratic, which has pretty much never happened during his rise to power and his stay atop the ATP ladder. Also, sometime since the switch, he's been adding weight under the bumper guard (polarizing the racket, making it less stable as I mentioned). You put two and two together and you can logically infer these points that I have mentioned. If he stayed with a stock K90 (with tight quality control), he probably would still be playing as well as he did in 2006 (and still won the French Open without sacrificing the 2008 Australian Open, 2008 Wimbledon, and the recent 2009 US Open). But either way, he'd still get crushed by Nadal. Polarizing your racket only makes it worse against him. But at least he generates a heavier ball and still dominates a majority of the tour.
[K]Six.One Tour (3) 367.5 grams 31.7 cm balance.
Mains: Babolat/Wilson Natural Gut @ 23 kg // Crosses: Luxilon Alu Power Rough @ 21.5 kg