Oh, Rubbish
Overscan is purely trivial, allowing a few more pixels in each direction,
which doesn't even work with some display screens. No, the Amiga isn't
limited to 640 x 400 x 12 bits, it cannot even perform a 640 x 400 x 12 bit
screen. Four more boards to provide 24 bits of color, but that would be
limited in the sense that there is no way for an end user to make their
software library use it.