I've stopped at 1080 for now because it was working without boostign voltageand i ran out of time for further testing.
One guide i found (not the OCuK forum one) said when using afterburner to not only ramp the power limit all the way up to start with but also the voltage, I didn't follow this advice (despite being from a reputable site and a top google hit) because that seems not quite right!
Ramping the power limit up to max is ok.
Ramping the voltage up to max is bad because it makes the GPU hotter and can actually reduce the max oc available.