With doing the comparison again of Linux 3.15 stable against Linux 3.16 Git, in a number of tests the performance was measurably slower with the newer code still under development. In a majority of the tests the performance was about the same, but in none of the cases was there any significant performance gains for this system.
All of the benchmark results, system details, logs, etc, are available via 1406227-KH-LINUX316K91. Worth noting is that Linux 3.16 is the first kernel to support the Intel P-State governor with this Devil's Canyon system so between the kernels there is a switch from ACPI CPUfreq with its ondemand governor as the default to Intel P-State performance being the new default scaling governor for this i7-4790K system.
The Apache performance seemed to be incredibly affected by whatever is currently going on with Linux 3.16.
Again, find all of the data on OpenBenchmarking.org. You can reproduce these kernel tests with the Phoronix Test Suite.