The Kindle 4 also supports KF8 and you are forced to use the publisher font with no way of choosing another. I believe it's the same way on the Kindle Fire. I can check later tonight and get back with you.
I bought my first KF8 the other day and the font used looks horrible on the Kindle 4. What in the world was they thinking?
Big spaced words along with the line space bug just looks unreadable.
That line space bug is one reason I stick to mobi.
Mantano makes you use the publishers fonts, color and background as well if it specified in the style sheet. This is why I started stripping the epubs as they render very differently than on ADE.