The key to run NF2 above 200 MHz is the motherboard and the north bridge chip and its memory controller in particular. The natural out of the box FSB ranges from 180 - 210 MHz (without vmod). Many have success in getting FSB to 220 MHz by doing vdd mod on the NB.
I have done some tests, even you have good working memory (PC3500) and good MB that the FSB can run up to 230 MHz (in ASYNC) per se, once you put the dual channel memory in, up the FSB in SYNC to somewhere between 195-210 MHz (depend on the MB), and pump memory intensive data such as 3DMark through the NB, the system will crash. The system is totally normal if there is NO memory intensive application running.
This means that the NB will fail at certain memory bandwidth (not just the magnitude of FSB MHz). Repeat, the problem is NOT the memory and FSB itself. It will fail even you have $400 PC5000 module !!!
Back to memory, IMHO, any pair of memory will have a few MHz mismatch for a given ras/cas timing. Unless you do it in a lab with a large sample to choose from and special setup, then you can match them to within certain % of frequency and timing.
Put it that way, even if Corsair hand picked and tested the TWIN module, it only guarantees that it will run at 200 MHz 6-2-2-2. Many memory module with 5ns DRAM chip will be able to do that without any problem, some may be even 30% cheaper and overclock few %.
Do you think Corsair will spend lot of time (30 min-1 hour ?) on each TWIN pair to run memory intensive applications such as memtest86, prime95, 3DMark, ... on that pair to ensure it won't crash with the NB in its A7N8X testbed. If not, after you get the TWIN pair, it may fail on you due to the NB problem mentioned above.
IMHO, if you want to spend that 30% more money, it only guarantees you at 200 MHz (system booting, booting Windows, non memory intensive applications, ... ???? I love to know how they guarantee these, ...) if your MB can do 200 MHz, and NOT guarantee your NB and running memory intensive applications, .... Further, if for any reason, if one stick fails, would they RMA both TWIN for you. For me, I consider non-TWIN will do as well. Many PC3200 module (even 30% cheaper) will do as well as expensive memory (if the NB allows). I would use that money to spend on other parts. Or get a true 3500/3700 (non-overclocked) if you think the MB can go to 220+ MHz.
Also agree that 8RDA+ overclocks better than A7N8X because the default higher adjustable Vcore and Vdimm. Also it is easier to do vmod on the NB than A7N8X (which is totally doable) due to component size and layout, ...
But on the other hand, A7N8X is richer in features, it supports on board serial RAID (though hard to get those drives) and has dual NIC which is good for file/internet sharing by just plugging in a laptop.