I Believe that whether you're in the BMus (Perf.), BMusStudies or BABMusStudies, if you're doing a study at Major level, then the lesson times and assessments are Exactly the same across the degree structures. Likewise at Minor level, if you're enrolled in an instrument study at minor level, you will complete the same work relevant to that instrument whether you're in the BMusStudies, BABMusStudies or BMus (Perf.). Students at Minor level have less lesson time and less assessments than those at Major level though.
As for doing a BABMusStudies or a BMusStudies instead of a BMus (Perf.), the idea is that each degree structure has a different focus. The BMusStudies has less subject requirements and is a year shorter, but this is not because it's an inferior degree, but because it provides a more general focus with greater emphasis on areas other than a student's major study. The BABMusStudies takes this even further, with the Arts emphasis on the humanities complementing the study of Music.