if we aren't talking about admissions, and just performance of students with and without degrees, i'd go with the students with a bs.
most of the schools i called prefer a student to have a bs. imo its because it gives the school peace of mind that the student will be able to handle the pace of pharm school. while taking pre req science classes may not be that difficult, stack them all on top of each other in one semester/quarter, and the difficulty shoots up. Most students with a BS have had 3 or 4 science courses stacked in one quarter, upper division included.
i think the school just wants to know if you can handle taking 4 difficult science courses all at once, something many of the students without a bs may not have done.
thats the only advantage i see to having a bs and how it would affect a student's performance in pharm school. other than that, pre reqs are there for a reason, that reason being that is what the school wants you to know, if you have them and are able to handle the pace, you should be fine. If you';ve never taken a full load of science courses, you might have a rough time adjusting, but nothing too serious.