Text Size 1 (smallest):
The letterforms and resulting spacing are unchanged as far as I can tell for the default style. I can overlay before and after shots exactly, and they match pixel for pixel (Before and After, respectively):
However, when displaying text with a size attribute applied (e.g. titles/section headings), Kindle seems to select from one of the larger text sizes, which have been enhanced (subtly, but the serifs on "E" are cleaner and the crossbar on "A" is all black instead of half gray):
Text Size 2:
Here we start to see the first real changes. The horizontal strokes and serifs are darker (note the vowels 'a' 'e' 'o' especially), dots in 'i' and 'j' are completely black and clear, periods are darker and crisper, the base serifs on letters like 'i' 'I' 'f' are symmetric, whereas before they were not. This results in slightly tighter letter spacing for some combinations (e.g. 'if') and a more pleasing 'rhythm':
Text Size 3:
Again, we see improvements along the lines seen in Size 2:
Text Size 4:
...and again, same thing in Size 4, though the differences are getting increasingly subtle and less significant ('g' 'o' and 'm' look better, though the i-dot is arguably a regression).
Text Size 5 and 6:
I could detect no changes to any of the characters used in my sample. The only attributed text I had in my sample (title text) looks slightly different, but not significantly so.
Yes, they did something to sizes 2, 3, and 4, and I would pronounce it noticeably 'better', particularly in sizes 2 and 3. And at least the text without attributes (bold, italic, size etc.) in sizes 1, 5, and 6 is exactly the same as before. It seems unlikely that they can improve this particular font beyond this (I think this is at least their second attempt to do so).
Readers with specialized requirements (or preferences) will unfortunately have to continue to hack in an alternate font.
I would recommend that Amazon either supply one or two alternate fonts (some sans serif font in particular) or better yet, make it possible to drop in a replacement without hacking, since they will never pick the right one for everybody. And for extra credit, provide an option to override Topaz fonts (though the results of this will not always be pretty, given there could be several embedded fonts in use). And in case I haven't said it yet today, they need to add display support for Unicode, or at least for Cyrillic, Chinese/Japanese/Korean, which as hacks have demonstrated, can be used without any other software changes (we'll worry about readability and text entry of those some other day...).