Do CPUs downclock due to ram ?

No

Are you asking if your CPU will run at 1600 MHz? No it won't. I believe AMD processors only support memory speeds up to 1333 MHz though so your 1600 MHz RAM will only run at 1333 MHz. If you bought an Intel CPU I believe the RAM would run at its default of 1600 MHz due to a faster controller. RAM speeds don't matter except in certain specific instances anyway (like with onboard GPUs). Also, look at benchmarks. That 4.0 GHz CPU will not outperform Intel's i5s and i7s even with its higher clockspeed.