You're really missing some of the basic points... Apple states the Mac Mini 2012 is only capable using 1600MHz RAM. The 2011 Mac Mini is only capable using 1333MHz ram, but works just fine with both using RAM at 1600MHz and 1866MHz, and actually running the RAM at those frequencies.
My own MacBook White Early 2009 that the specifications states should use 667MHz DDR2 RAM runs the current RAM specced for 800MHz just fine at 800MHz.
Of course, it's out of the original specs, and hence it is not very certain that it will be running RAM at those frequencies, which is also why I suspected the end result, as it actually turned out, that the 2012 Mini runs the RAM at 1600MHz.
In other words, there is no difference in practice, at least frequency wise, between the 1600 specced Corsair and the 1866 specced one. Timing wise, it could be that the 1866MHz one has a little edge over the 1600MHz one when it is being run at 1600MHz, but knowing what I do today I would not pay extra for the 1866MHz model.
But, until someone actually had tested it, none in here could really say what the outcome would be.
If it would've been running at 1866MHz, there could be a good performance increase.
Now, as for my case, I fell back to the Kingston HyperX @1600MHz, with CL9 timing.
It is very possible that a future EFI update may unlock the option of running RAM at higher frequencies, especially since the CPU itself which does contain the memory controller is capable of it, but I wouldn't keep my fingers crossed.
A good recommendation to you, it is always a good idea to let new memory run over night using either Memtest86 or Memtest86+. I would recommend you to do the same.
If you have a Bootcamp partition or know anyone with a Windows machine, there is a easy tool to create a bootable USB drive for you at
http://memtest.org/
When I used the 1866MHz Vengeance, I successfully completed all of the tests, and had no visible or otherwise detectable issues from within OS X. Running Memtest86+ over night did bring these issues to my attention however.
Running Memtest86 won't really guarantee that your RAM modules are okay, that is impossible, but it will be able to show you that they are at least not showing signs of failure over the period you run them (hence, the longer the better).
It might been possible to detect it with Apple Hardware Test in extended mode for a similar amount of time, but my weapon of choice is Memtest86+ due to the good information it provides.