Are you using Precision X to add voltage? Have you checked your voltage using GPU-Z?
Start GPU-Z, then click the sensors tab at the top. Then go down to VDDC and click to show highest reading. Then start a game or benchmark and let it run in the background. End the app after a few minutes and check GPU-Z to see the highest voltage measured. This works with Precision X adding voltage, does not work with GTX OC Tool since that voltage can't be measured with software.