I've compiled a few things we did that might help people to get higher clocks on the Lightning:
http://elmor.se/view/108/howto_overc...ghtning_on_ln2
From mine and Henrys findings, the most important things are to use good thermal paste (OCZ Freeze) in combination with keeping the pcb warm. This is basically what allowed us to run 1400 core on these cards. If the thermalpaste looses some of it's conductance at low temperatures, it will have huge impact since the heat emitted from the GPU is quite severe.