This patch disables radeon VRAM amount overriding with VideoRAM option in xorg.conf. This should be good, but there are texture problems with some OpenGL applications (celestia, for example) who can be fixed by using less RAM that the amount actually installed, and now this is not possible in Gentoo.
shouldn't those problems be reported to the DRI driver maintainers so they can be fixed properly?
Indeed. Please file upstream bugs at bugs.freedesktop.org when needed.