I finally found another nice Serif font. I am so picky, I use this one on Pocket Pro when I had it. I added some weight and it looks really nice on this Kindle. There are two versions It's called Gentium Basic & Gentium Book Basic. It's under the same license as Charis SIL so I think I can share if you like.
PM me if you like to try it.
Tested on the KF8 and it looks fine as far as I can tell.