Sony's studios may be using MPEG2, but BR as a standard supports Mpeg2, VC-1, and Mpeg4 (h.264). Additionally, the average bitrate for DVD is around 6-8... meanwhile BR is going to be using 35-40 for its Mpeg2, or whatever bitrate they want to use for h.264/VC-1. So far, only Sony's studios have said they'll be using Mpeg2... Unless you want to provide some information that all studios _are_ using Mpeg2 (hell, or any studios besides Sony's), I suggest you don't assume things.
It's likely that the studios using Mpeg4 won't be using 35-40mbit streams any ways, as it just isn't necessary to get quality picture.
It's also likely that Sony's reason for using Mpeg2 stem from it being cheaper to license, and cheaper to produce something on it (as the tools are far more mature and they have experience with it). It being on BR alone will reduce the piracy (assuming that the AACS stuff even gets cracked any time soon) -- anyone who _can_ actually rip a BR/HD-DVD movie isn't likely to care that the movie is 25gb vs whatever it takes up using h.264. I doubt we'll see people ripping the movies any time soon.