Ehhh, more MP actually improves dynamic range to some extent by increasing total signal, although the D700 is significantly ahead of the 5D2 in DR. Remember, you're evaluating image quality, not pixel quality, so low quality pixels can be offset by simply having more of them.
Re: 15mp being the limit, whiners were complaining that 50D was worse than 40D and that 12mp was the limit for reasonable performance, and now Nikon is about to follow suit into 16mp. For FF, Nikon D3x at 24 mp actually outperformed 1Ds3 at 21mp, so it seems as though it's more a matter of technology than hard physical limitations.
Remember, on compacts, the G11 dropped back from the G10's 15 MP, but scaled up to APS-C formats pixel density would be equivalent to 70 MP.