When we test RAM modules we tend to say that the speed of the RAM has no significant impact on the performance of Intel systems. Intel's smart cache and memory controller make it so the CPU almost always retrieves the necessary data from the memory beforehand. No matter if its DDR3-1333, DDR3-1600 or DDR3-1866 RAM, we've never seen more than a 5 percent difference in performance aside from the specific memory benchmarks. In games and other standard software you don't notice any difference.
And while this is true for Intel processors, it's not necessarily the case for AMD chips, more specifically APUs with integrated graphics. With these processors the memory bus is shared between the CPU and GPU cores, which means the memory is under more load. AMD also hasn't developed prefetching algorithms and other things that can mask the speed of the RAM to the extent Intel has.
In order to find out exactly what the influence of RAM speed is on AMD processors, we put an AMD A10 5800K processor on an ASUS F2A85-V Pro motherboard and ran a number of benchmarks at various memory speeds. We used Kingston HyperX Beast DDR3-2400 CL11 modules that are able to run at all speeds.
The performance differences for 3D benchmarks and games, which really tax the integrated GPU, are significant. In 3DMark 11, DDR3-1866 offers almost a 12 per cent increase in performance. In 3DMark Fire Strike, it's almost 18 per cent. In games it's even more, more than 20% in Tomb Raider, almost 25 per cent in BioShock and Dirt and almost 30 per cent in Hitman Absolution. You can have at the results and read the rest of the Mini-preview: the effect of memory speed on AMD Trinity APUs on Hardware.info.