Not sure what your point is, it is pretty clear that the hd 6450 are as far as power house as can be.
Still I'll let you make the calculation about the difference in pixel rendered between 1280x1024 and 880x720p. there is a massive difference( a tad more than a power of 2), I wonder how "well" the suckiest" GPU AMD sells perform at that setting.
Anyway that has nothing to do with Nintendo design choices, whatever its merits or lacking the hd6450 doesn't have what it takes to power the extra screen.