I asked Roy, and this is what he told me:
[I]2880 dpi uses just one size dot on most printers. 1440 typically uses 3 possible size.
2880 is actually usually 2880x1440 and 1440 is usually 1440x720. So there is a 4x
difference in number of dots.
For the amount of ink, my assumption would be that for the same looking print you’ll
use the same amount of ink regardless of the resolution. Curves are made for a specific
resolution so another res might be darker or lighter. The downside of 1440 would be
speed, sharpness, etc not amount of ink. [/I]