I believe that makes ini edits that only apply after a game restart.
I’ve found that the main page FOV Enhancement slider only appears after you turn directVR off in the profile. This is probably to prevent conflicts.
See if this enables the Enhancement slider for you, but note that this method can introduce graphical glitches, so should be used as a last resort. Always best to first configure game files directly, use image zoom as much as tolerable, and lastly push a bit further with Enhancement if needed.
For supported games, directVR should do all this for you though.