As part of resampling, the diffusion dithering, more so than the ink dot size, that occurs spreads the location of a printed pixel a bit. This creates a low pass filtering effect similar to the "zero hold" low pass filtering that occurs with a DTOA output is held constant between clocks. On the "zero hold" DTOA it produces about a 4 dB drop at Nyquist. The dithering, or pixel spreading, is a somewhat larger effect that occurs but can also be measured.
I ran an experiment with Canson Rag Photo on my 9500II and there is about a 5 dB drop off at 250 PPI and 2 dB at 150 PPI when printing 600 PPI images with the finest setting. It seems possible that this characteristic could be measured for a given printer and an appropriate sharpening filter applied. There are limits to such a filter. Pixels that would be pushed beyond the media white or black would have to be spread out so one couldn't completely compensate though I believe that rare in most real photos.
I'm also curious whether glossy is better.