The only time you'll really see a difference in 16 bit is when you're looking at a texture with a color gradient (i.e. one edge of a wall is brightly lit, while the other end is not). Instead of smoothly transitioning from light to dark, you'll see the transition in "bands" of color, ranging from light to dark. This is because 16-bit color displays far fewer colors than 32-bit, while 32-bit color needs more decimal-point precision (floating-point precision?), which is why it requires more resources.