DVD Studio Pro doesn't encode MPEG2 video at all. To use any sort of video for DVD Studio Pro, it has to already have been encoded before DVD Studio Pro will even recognize it. Thus, in a way you could say it's flexible for bitrates, in that the bitrate at which the movie is coded is wholly up to you.
Fortunately, DVD Studio Pro gives you a licence for the QuickTime Pro MPEG 2 encoder plug-in.