As far as how SNA impacts 3D performance, since it has affected some OpenGL performance results in the past, the situation is currently mixed. SNA for Ironlake did cause the OpenArena frame-rate to go up while for the Core i5 2400S the frame-rate dropped and for the i5-2520M the performance was unchanged. Regardless, for Sandy Bridge hardware, Mesa 8.0 is a huge improvement over Mesa 7.11 as earlier Phoronix benchmarks have illustrated.
The World of Padman performance was similar.
The Nexuiz graphics performance was largely unchanged for the Sandy Bridge hardware across the three tested software configurations.
As you can see from the results, overall Sandy Bridge New Acceleration (SNA) does offer some compelling performance improvements for the 2D Linux desktop. However, this is not without some regressions, some of which have recently come up as SNA continues to be developed very actively by Chris Wilson. As said by Chris when seeing these results early, "Judging by those results, I fear performance has regressed considerably on those systems." Chris is currently investigating the regressions found by these SNA results. (Update: Looks like there's some Intel DRM bugs being hit during the 2D usage. More SNA results will surely come when the Linux 3.4 DRM is ready.)
SNA is still looking very hopeful and if you wish to try it out the xf86-video-intel DDX driver needs to be built with the "--enable-sna" switch. Ideally if testing SNA the Git snapshot is the best due to its very active development state. As to whether we will see SNA-by-default anytime soon, stay tuned to Phoronix for the latest developments.