We may be arguing apples and oranges. I will say that putting weights in a BC is proving the arguement. You are affecting trim by positioning weights within the framework of the vest. As to the bp/w, repositioning the backplate or the bands on the tanks will affect the trim one way or the other. Note: Just adding weights to the tank may do more to adversely affect your trim in the water, not to mention the additional work required to move in the water column.
Of course body composition is the determining factor in all this.