Prism and ARTPOP doing well
I think Prism might be able to do at least 5M, depends on how her singles fare. ARTPOP will probably end up with 3M or 4M at best, also depending on the choices this era.
I don't think anyone was saying that Katy was a stronger seller than Beyoncé, but it's not out of reach to assume that PRISM and Beyoncé will probably match each other in sales.