AMD's Michel Dänzer wrote on the xorg-devel list an interesting message on Monday: "This is rather interesting news: So far the assumption has been that glamor cannot work at all with current xserver. However, I cannot reproduce the problem I reported a while ago even with xserver 1.13.0, let alone newer versions, though I still can with the pre-1.13 Git snapshot I reported the problem for. Apparently the problem was fixed before the final 1.13.0 release. Was I really the only one who ever tried this, and everyone else just took my word for it? :\"
So surprisingly, GLAMOR apparently works on the current X.Org Server but it wasn't realized for months... Many months. X.Org Server 1.13.0 was released last September. But sadly, the GLAMOR acceleration library isn't vigorously developed nor used, so it's surprising compatibility went unnoticed.
RadeonSI, AMD HD 7000 series open-source driver benchmarks will hopefully be here soon on Phoronix.
For information on how to setup the AMD RadeonSI open-source driver, see this forum guide.