The focal length increase from 200 (your 70-200/2.8) to 300 is only 50%. So I don't think you gain much image size at the same aperture. Similarly, the focal length increase from 400 (your 100-400) to 500 is even smaller, only 25%. So not justifiable from image size. Only from image quality or lens speed would I get the 500. BTW, how large are you printing or displaying?