Jordan Justen of Intel published sixteen patches of "Gen6" render surface state changes to allow layered rendering to work for Sandy Bridge. These patches also fix 10 Piglit tests plus FreeDesktop.org Bug 56127, a report that was opened in late 2012. Back in October of 2012 a regression of the Unigine Sanctuary performance being reduced by 98% and Nexuiz was reported. The issue was bisected and back in 2013 it was attributed that Jordan's layered rendering work should fix the problem.
Jordan's patches for this Sandy Bridge improvement are now under a "request for comments" on the Mesa mailing list. Hopefully this work will be merged for Mesa 10.3 when released in about three months time.