I am soon to be in the market for a laptop with a high-dpi screen: considering we are treating 300dpi as laser-quality printouts (or at least we did in the early 1990s :), I am a bit saddened that you didn't simply find 240dpi to look perfect with neither hinting nor subpixel text positioning.
Any screenshots included in the document don't do the actual renderings any justice since we are mostly looking at them on lowly 110dpi-140dpi screens :)
Hi Danilo, two things you have to consider: First of all also 300dpi laser prints do not look perfect for very small fonts. Current smartphones are often much closer to the viewers eyes than sheets of paper. As they have less space, some fonts are rather small.
Second (and more important): The main topic of the text is, how to enshure backwards compatiblity. This means, that your must abstract to a lower resolution than the display is capable. Now the problem is how to abstract while keeping the most advantage of the high display resolution.
To the screenshots: They might not represent a high-res screen, but in case the space distribution looks very odd, you will see this in high-res as well. Smaller offsets might be only important for purists, but if you care, they are still visible.