OK, Where to begin, i do not have much time to write this post otherwise i would have written an entire essay full of references, but i will try to make my point briefly (almost)
Maybe this argument is specific to the European market where i get my prices from, but this is all based on the fact that 1600 CL9 modules here in Europe are not cheap and the difference between them and the 2133s or 2400s in price is overall negligible some 8-10 Euros difference which is far from the difference you pay going from 8GB to 16GB or from 16 to 32 etc... and you can not also normally use that extra money to better any other component in the system, get an SSD, a better GPU or else..
I have initially followed the advice of 1600Mhz is all you need and got 1600s for my Haswell build after being on 2400 with Ivy and then ended up overclocking my 1600 to 2133 CL10, so i have used systems with 1600, 2133 and 2400 and have tested the differences myself, i ended up returning the 1600s that i got for my Haswell build and ordered the TridentX 2400 CL10 instead which i have not received yet.
Here is my reasoning for doing so.
1) IGP performance is the most significant and noticeable of all these, as going from 1600 to 2400 changes the IGP performance very significantly and the difference is like day and night, now as to why anyone wants to use their IGP when they have a high-end dedicated card has many applications, for one having 2400 on an HD4600 Graphics that comes with Haswell makes a lot of AAA game playable on IGP in case you RMA your Graphics card or replace it, you can still use the IGP while you wait, also if you use that software that splits your computer into two, you can have two PCs each running with its own GPU and you can even play multiplayer games on those, also if you use VirtuMVP, the IGP part of the pool will be able to output more performance, and the difference in performance in this case between 1600 and 2400 is like 30+% and is directly linked to the speed of the RAM which makes a lot of unplayable games, playable on IGP as you get up to 10-15 FPS with faster memory!
Even Intel officially recognize that fact according to their XMP page found here
http://www.intel.com/content/www/us/en/gaming/gaming-computers/intel-extreme-memory-profile-xmp.html In this page there is a linked PDF list showing all officially certified speeds and voltages for all their K version CPUs and is updated frequently which answers yours question of what is the best performing RAM modules for each different K version CPU. The list also has certified brands, modules and motherboards, it can be found here
http://www.intel.com/content/www/us/en/gaming/xmp-for-core-processors.html
2) Compression performance of things like WinRAR increases by a nice margin and is proven by all reviews, system boot time increase noticeably when using faster RAM, even by a second or two, it is still good given the very tiny price you pay to achieve that.
3) DirectX 11.2 that comes with Windows 8.1 and is also used in the Xbox ONE claims to be able to utilize the system RAM to provide a Swap RAM for the video card if its vRAM Is exhausted, the same type of implementation that they did in Xbox ONE which have 2133Mhz RAM, and my GPU comes with 3GB of vRAM, i game on 3 monitors 4K resolution and some games already in the market do require more than 3GB of vRAM already and i have to lower shadows or AA to be able to fit in the RAM, with DX11.2 and Next-Gen games, maybe system RAM will become useful when games require more vRAM than what you have in your Graphics card and when this happens you better have the fastest RAM possible, at least match Xbox design and have 2133Mhz.
4) I can not locate the exact reference thread now, but when i was researching this issue, i have read 100s of posts and one of them was for a guy in overclock.net who have done massive testing for months for RAM from 1333 till 2666 with all sorts of Graphics cards from both vendors and he claim in a very objective analysis that games do see up to 13FPS increase with faster ram even when using a dedicated GPU and this finding match one of the things in the thread you posted of a guy saying that PCI-E performance is enhanced by faster RAM.
5) Computers are different than consoles that they are multi-purpose devices so claiming that someone will be using his system for only Gaming all the time and thus they should stick with 1600Mhz does not make sense to me, what if this person decides he wants to do some video editing or Photoshop or whatever that see performance increase with faster RAM? would they have to settle for slower performance just because the initial purpose of the machine was gaming?
6) even 2133@CL11 or 2400@CL11 provides much better speed and latency than 1600@CL9 after personally testing it, as it provides from 3 to 4 GB/s extra bandwidth, and up to 10ns better latency, even going from 2133@CL10 to 2133@CL11 does not effect real response time of the RAM a lot if at all according to my own testing, faster is always better.
All this is valid as long as the price difference between 1600 and 2000+ modules is not significant enough to matter more in other areas which is perfectly the case in the European market.
For all the reasons above i decided to go with the faster RAM than 1600Mhz and think that the (stick with 1600 RAM) is a bad advice these days, one that i have personally been a victim of