Although this has been well answered, I'm going to pitch in with a clarification. You wrote "If My RAM (1600MHz, 12Gb ChannelA, 8Gb ChannelB) is running in dual channel mode so the speed of each channel is 800MHz...." Dual-channel mode does not divide the speed of each memory stick in two, somehow sharing the bandwidth between the two sticks. It runs the two sticks at their rated speed, theoretically doubling your transfer speed, but actually giving a somewhat lower boost.
Also, memory runs in dual-channel mode when you have the same memory configuration in both channels. You don't; you stated 12 GB in one channel and 8 GB in the other. Depending on the motherboard, you will at best get dual-channel mode out if 16 GB of memory...