If you want to run your 1600MHz RAM at spec on that multiplier you will need to increase the Bclk to 200, you are currently using the 8x RAM multiplier, when your RAM was very high at 172 Bclk you were on the 10x RAM multiplier.
The other option is to OC your RAM, a good 1600MHz kit should hit 1720MHz, you may have to loosen your timings some though. The last case is to lower your Bclk to 160 and use the 10x RAM multi for 1600MHz RAM but a lower CPU OC.
4.2GHz is a very popular OC at 200 Bclk with a 21x CPU multi(4200MHz) and an 8x RAM multi(1600MHz).
I found the sweet spot on my 920 with a 200 bclk also. You can use the multi to go from 3.6-4.2 with ease. Video editing I would oc around 3.8 with HT. That's my preference with animation on my 920 anyways..