the older GPUs dont like mutch undervoltage :/ (tested in my R9 270X, R9 280X and R9 290) BUT in the RX470/480 cards the undervoltage do miracles! mutch lower temps and low powerdraw! im at 1100Core with 950mV some of them go very well to 850mV but in my case is more stable at 950mV (core and mems)