It is also important to keep the memory in original pairs in the same color slots as the speeds of different matched modules will be slightly different even at the same rated speed. If you have that under control, then it might help to swap red and black slots. Don't get too hung up on fast ram, it does not make much difference.
When I put a second set in they were 10% faster, not sure why.. I could not be bother swapping to find out, but one set of slots (2,4 for single pair config) might be slower for XMP memory. Either that or my second matched rams were faster.
You could use this to work it out and maybe work out the best match..
http://www.memtest.org/