The new Corsair CL9 memory on amazon uses a different voltage than the stock memory (1.35v vs 1.50v).
If you fill all the slots with the Amazon Corsair memory you'll be fine (32GB) and run at 1600Mhz. But if you want to mix and match (with the stock Apple memory) you have to buy Crucial memory for everything to run in sync and at 1600Mhz.
Personally, what I might do and think is the best solution, just get 16GB of Crucial. That makes 24GB of RAM combined with the Stock 8GB RAM. This is more than anybody really needs right now.