I underclocked my toxic core and other cards mem to 1500. oc'd core on asus and saph vapor x.. ended up gaining about 130 h/s. 4 card rig at 1933 h/s.
What memory do your cards have, Hynix or Elpida?
i think his card memory is hynix all. because elpida 280x max speed about 400h/s/card