I Accidentally discovered this... borderline Glitch
First make sure the FPS counter is ON, so you can monitor the FPS. I believe it's in the main DeoVR settings, when you can see the Grid.
Then make sure 120Hz is selected for the refresh rate.
Make sure Sharpness is set to zero and save it as Default. Anything higher and it will only play at 60FPS.
Now here is where it gets weird... Any scene that is played from ANY Interactive Playlist... like from here: https://www.sexlikereal.com/interactive
... now will play at 120FPS!
That same scene played ANY other way only plays at 60FPS!
So, for some strange reason the "Interactive Playlist" makes it possible to play All scenes with AI PT on the Q3 at 120 FPS with noticeable Much smoother playback.
Can Other Q3 Users Confirm This Works?
...Also, Quest Game Tuner not required