The CacheBench write speeds appears to drop significantly under GCC 7.
SciMark2 is running at its prime with GCC 7.
SciMark2's win with GCC 7 comes down to Dense LU Matrix Factorization being faster than the three previous GCC releases.
In many of the benchmarks run, the performance changes were very minor.
GraphicsMagick is perhaps a little bit faster with GCC 7.