My temps are now way down. Hard to get either GPU to go above 31/32 under stress! With the stock cooling I could hit 70 easily...
Managed to overclock to 800 (from 600) without any problems as a result using RivaTuner.
You need to dedicate an hour to stripping down your GX2 when the block arrives. Take your time and follow the guide and you will be fine. Worth making sure you disassemble carfully, and take time with the TIM etc. Well worth doing the job right.