Asus Rampage IV Extreme and Memory

If the RAM is not on the QVL that means asus does not test it yet. But it does not mean the RAM is not compatible with MB.
In other hand you will OC the 2133MHz RAM if you buy that. And you can read the review in the link you will see even the 1600MHz RAM that has same performance as the the 2133MHz RAM, but bandwidth and latency matter. So I will recommend to get the 1866MHz one because it has better latency.
http://www.tomshardware.com/reviews/memory-bandwidth-latency-gaming,3409.html