I've been aware of this since the time I was playing with 8 KingstonX BH-5 sticks and my old nf7-s trying to make 2 of them work good in Dual Channel.
I finally came up with a set that were doing 240Mhz 2-2-2-5 @3.04V stable in memtest. The funniest part was that if you would mix both sticks (take #1 stick and drop it in the slot of #3, than I was ending with errors like no tomorrows in memtest #5