Kingston HyperX Red - 8GB (2x4GB) DDR3 1600MHz Memory Kit Review (KHX16C9B1RK2/8) - Benchmarks - PCMark7
PCMark7 is a "real-world" benchmark which runs an array of real-world tasks, such as multiple-tab web browsing, file copying, reading, and writing, gaming, and a large variety of other tasks which are meant to give your system a rating as it pertains to actual usage, and not engineered synthetic benchmarks. This benchmark will give us a good idea of the actual difference we're likely to see at these different settings.
As PCMark7 shows, we see similar trends as AIDA64 and Sandra, but the total system difference is only 3.5% better at 1600 MHz and 4.5% better at 1866 MHz. The 3.5% increase could be noticeable in very computation-intensive environments such as rendering or running simulations, but on average that 3-4% will be hard to notice. This has been a general symptom of memory since it has become extremely affordable and we generally have an excess of RAM, so the big gains we saw from increased RAM capacity isn't a story realized by many these days.