Could be you are reaching a sweet spot and another 100MHz require a hefty voltage bump and maybe also a big temp increase. I would first try the below:
Question: Is there an AVX offset in the bios? If so set that one to 2, this means if an AVX program is launched, it will reduce the cpu mulitplier by 2x, all other non AVX programs will run flatout.
We all delid our Skylake CPUs and replace the thermal paste on the DIE, temps could drop like up to 20°C in best case.
No mattter what the Bequiet specs tell you the a top flow cooler is never as good as a high end Tower cooler. Always Monitor your temps with eg Realtemp
and no don't touch the bclock, no need...