[GeoffR]

For anyone sceptical that you would be able to see differences between a 300 dpi and 600 dpi or higher screen, here is an experiment you can try using your current ereader:

Attached is a simple ePub book that has two pages. On the first page are two squares, the inner thick square is drawn in a two-pixel-wide stroke and the outer thin square drawn in a one-pixel-wide stroke. The second page has the same but with a thin inner square and thick outer square.

I can read comfortably holding my ereader at 25cm from my eye, so I tried at increasing distances of 50cm, 1m, 2m, 4m, and 8m to see if I could still see the squares and tell which was the thick one and which the thin. The light conditions were overcast near midday.

Up to and including 4m I could still easily see the squares and distinguish which one is thick and which thin. At 8m I could no longer see the thin square.

My ereader has a 212dpi pearl screen, so at 4m I calculate the squares appear the same size to my eye as they would on a 16x212 = 3392dpi screen held at 25cm, and at 8m as they would on a 32x212 = 6784dpi screen.

Edit: I should just add in conclusion: Since a one-pixel-wide line on a 1200dpi screen is the same width as a two-pixel-wide line on a 2400dpi screen, I think from what I found above that I would be able to see the difference in thickness of a one-pixel-wide line at normal reading distance as the screen density increases from 300dpi to 600dpi, 1200dpi, 2400dpi. Maybe 4800dpi.