Have you got GPU scaling enabled in the Radeon Settings / Display settings? If yes then turn it off.
Overscan / underscan compensation and GPU scaling are two different technologies and both don't need to be enabled.
The other option is to sort out why you are not getting 1:1 resolution in the first place. Normally that would be caused by the wrong setttings on the TV / Monitor. It can even be caused by for example running at 59Hz instead of 60Hz.