You are right, i also did not gain any FPS. But look at the visuals!!! They get much sharper!! So if you want to gain FPS, just lower your in-Game Render Scale from 80% down to 60% and there is your gain.
Or in other words: you reached your goal of 100% OXR and 100% inGame with your current setting of 100% OXR and 80% inGame. Please try and compare the visuals - and let us know.