Taylor Trotter :
the new kaveri APUs benefit directly from higher frequency due to the bandwidth being the bottleneck in APUs. 2133 performs best for the money with 2400 being unsupported by all but the best of boards and most APUs other than the 7xxx series unable to get above 1866mhz anyway. best choice for APUs is the 7850k with 2133mhz memory and a 250x.
Technically, you're mostly correct, but a couple of your facts need minor adjustments.... Yes, 2133 is the recommended RAM frequency for AMD APUs and Dual Graphics implementations, but there are other APUs OTHER than just the AMD "7xxx" series that can handle 2133mhz RAM natively without much trouble of overclocking the APU itself.
-- One APU that I can think of right now off the top of my head that's NOT a 7xxx series APU, that also supports 2133mhz RAM, is the AMD A10-6800K. It natively supports both 1866mhz AND 2133mhz RAM.
-- And you also said that you believed the "...best choice for APUs is the 7850k with 2133mhz memory and a 250x." Wellllllllll, if someone is wanting to run their R7-based APU(ie. A10-7700k, A10-7850k, etc.) in Dual Graphics mode in conjunction with a PCI-E discrete GPU, yes, 2133mhz is the sweet spot for the system RAM speed, but running a 250x GPU won't work. True, it would be a good card to pair up with an APU and disable the integrated graphics on the APU itself, but in order to successfully run AMD's Dual Graphics on these types of processors, ONLY the R7 240/R7 250 will work "properly." This isn't just me guessing either. It's well documented.... Also, I'm typing this from my desktop with the AMD Kaveri APU you mentioned, as well as G.Skill Sniper 2133 9-11-10-28 RAM, coupled successfully in Dual Graphics mode with a R7 250 GPU and it all works wonderfully together.
ps.. this reply isn't meant as criticism of your post; just as a helpful correction! =)