Interesting
I used the red slots and could run DDR3-2000 8-8-8-24 at Prime 95 until I stopped it. With the Cellshock sticks it didn't matter if black or red, frequency limit was in the low-mid 7xx MHz range as described above.
However even with the Crucial DIMMs I had memory loss (< 1 GiB available) at 2.0 V this time (Boot @ DDR3-1866/7-6-5-18 2T), so it seems we're really on the edge with D9GTx.