Only nerds will worry about wasted bits :-) As to claims of slowdown, they are due to unoptimized APIs and slow hardware, mostly an issues with older devices. My primary concern would be how it effects the battery, if the difference is not so much might as well have 16M color.

If you just present a picture, people are not going to be able to tell the difference, but if you are using graphics programs or something and it has a color selector of a row of the same color subtly shaded, some of the colors are going to look the same on a 64K screen.

The difference between 64K and 16M is slight, but not quite undetectable (at least I could notice the difference). http://www.easyrgb.com/index.php?X=RESO