This patch reassign some nodes to other group level, this only brings a 60% performance improvement for the group level 0 (for clay renders for example).
It also adds some more features, like transparent shadows which are activated by default in 2.78 with no way to deactivate them anymore. So this resolves a 10-15% speed regression.
Some new features were also added which brings a 140% speedup when rendering with micro displacement. This last change of course brings some more kernel recompilation. But even including the kernel compilation times (which are also greatly reduced), the speed gain is still very high. So only downside is that a few more MB are going to be taken on the HDD, but in times where a 3TB HDD cost 70€, it's not a real problem.
A one week test has been done on BA and no reproducible bug has been reported so far https://blenderartists.org/forum/showthread.php?407044-Improved-OpenCL-build-beta. Only one user reported problems while rendering with HDRI but nobody else could reproduce the bug and no file was submitted. You can also see the reported benchmark results there. Tested platforms from me and BA users where Linux and Windows with different drivers and graphic cards from GCN 1.0 generation to RX 480.
Good side effect are also that some users reported that it also solved some rendering problems on old generation compared to master and RC2.