One thing that I have to admit didn't occur to me when I began to regularly use prim breasts is adjusting my shape, save for running the breast size slider down to zero to avoid interfering with the prim breasts. That's arguably an oversight on my part.
Today I found a store, It Figures, which specializes in "shapes for the top-heavy". The shapes in question are voluptuous and look quite nice. Do check them out. The store I found is in the Titty Bar Mall; watch for updates if I find other locations.
UPDATE: There's also an It Figures store at Mammatus Mall.