1. Using the fastest system memory helps the APU, because the CPU and APU share the system memory.
He didn't say anything about APU. Read his post thoroughly and more carefully. He asked if RAM would affect discrete GPU.
2. In CFX mode the speed of the GPU matters very little.
That's exactly what I said. Difference between "GDDR 3" and "GDDR 5" video cards would be insignificant if CFXed with Trinity, but it will give boost nonetheless.
Everything else: well duh.
P.S. Also, about memory: the fastest memory should help more, but the problem is in that, the faster the memory is, the more unstable it is.
Thus getting ultra-fast memory if it's not supported by OS, is not a very good thing to do. Shenanigans may happen (and actually do happen quite in many cases).