AMD currently has RV740 cards running with 900Hz GDDR5 (that's 3600MHz effectively) in the lab. Exactly why they have chosen to use GDDR5 with a chip that is supposed to be mainstream is a bit hard to say, but it may just be to test it without memory bandwidth limitations. AMD were testing RV730 cards with much higher memory frequencies than those seen in stores. The RV740 silicon is A11 revision, board is B507/895 and the GDDR5 is Qimonda/IDGV1G-05A1F1C-40X. Whether the GDDR5 is justified or not is hard to say, but what if it is?