Ok so I just realised the GPUs aren't taking the OverdriveNtool settings apart from fan speed and memory clock. Core clock does whatever the fvck they want and so do the voltages. HWinfo64 shows 1.25V on the memory despite me having set it at 900mV... 
I think I'm going to give Wattman a go 
I think I'm going to give Wattman a go well it's not that simple. It actually takes the values...sometimes... ghm...and I'm yet to discover what are the circumstances.
Also this weird core curve is needed to bring memory above 800... it all looks weird