I haven't looked up memory scaling with Trinity APUs, but for Llano at stock speeds, there's diminishing returns beyond 1600 MHz. 1866 works well on them and is usually only slightly more expensive. I'm sure that changes a bit when the GPU is overclocked, which they tend to do well at.
I'd test it with mine, but the top memory multiplier on Llano results in the memory running at 1866 MHz, with the reference clock at 100 MHz. I'd rather keep AHCI turned on, so I can only increase that by a tiny amount.
Hopefully someone tosses those on a Trinity system and posts results