XPS 8930, quick question about ram

I am retiring a xps 8900 which has 2 8G ram sticks of 2133P. I was wondering if it is possible, or worthwhile, to add these sticks to my 8930, which has 2 8G 2666s. I have seen where 2133s are being sold for the 8930. The Dell memory configuration page says that 8930 can up to 2666, but doesn't say anything else about what is actually compatible. I have also read that if I put in the 2133s, that all 4 will run at the lower speed. I have the i5 configuration. 

Will