While LLVMpipe may be a different story, when using hardware-accelerated graphics drivers with the recently released Unity 6.8 desktop, the performance doesn't change much. For at least one driver, there's even a new OpenGL performance regression under certain workloads. Here's some test results of Unity 6.6 vs. Unity 6.8 on the Radeon and Nouveau drivers.
On two distinctly different systems, the Unity 6.6 and 6.8 releases were compared since Unity 6.8 is advertised as offering performance fixes. Most of the performance work was for improving the Ubuntu desktop's performance when running atop Gallium3D LLVMpipe, a.k.a. the software-accelerated fallback when no actual hardware graphics processor / driver is available. Unity 6.8 is said to be more usable this way. With the various graphics performance problems attributed to the Unity desktop and its Compiz compositing window manager, this 6.8 release was quickly benchmarked at Phoronix.
The first system was an Intel-based desktop with a Radeon HD 4650 graphics driver running Ubuntu 12.10 snapshot while with the mainline Linux 3.6 kernel, Mesa 9.0-devel, X.Org Server 1.13, and an xf86-video-ati Git snapshot from master. Swap buffers wait was disabled during testing on this system.