@haydnfan: Err, sorry, but no. Unless FW 4.1.0 is even more broken than FW 5.1.x, which I doubt...
So while I can confirm that *some* KF8 books have font sizes that differ (sometimes vastly) from their mobi6 counterpart, that's not the case for the majority of *my* archive

.
On the other hand, what *does* affect the display of *all* KF8 books is the line height/line spacing issue, which I mentioned earlier.