I wouldn't go too crazy trying to shoehorn a massive PSU into a shoebox sized case. With a modern IVB chip at 77W and powerful GPU's in the ~200W range, Anything more than 350W would seem to be gravy. I mean my first concern would be heat output of a 7970 inside a small case then power concerns.
I have an idea. Only make one case face, but ship the piece of aluminum that fills the hole smoothly. That way customers can choose ODD or no ODD.