This is related issue to this bug, however an independent one (http://bugs.gentoo.org/show_bug.cgi?id=62687).
I would kindly ask that same kind of possibility of chnaging Xdriver version as was present in Xfree ebuuild would be present in Xorg ebuild. Xfree ebuild alowed user to simply change (SISDRV_VER="") and enjoy having new driver compiled on their system. Xorg ebuild however does not have this imho great feature.
Another argument is that several issues that people may have with the driver have been resolved sinse Xorg official release (including driver 2004 08 20)
For non sis users it would create unnessesery increase in download but this could be avoided through USE flags for instance.
Since X.org releases more frequently than xfree so fixes are available sooner, I'm not interested in increasing the maintenance and support burden to Gentoo by doing this. There just isn't enough time.