This is a slightly adjusted 25.3 ebuild that will give you the 8.33.6 ati-drivers for Gentoo. Yes, it’s dirty. They aren’t in the main tree yet because they are considered broken, although it works just fine for me.
Download: ati-drivers-overlay-8.33.6.tar.bz2
Extract them to an overlay.
Update, the 8.33.6 drivers are in the mainline tree now, so you should use those instead of mine.