Downloaded the game just because I was curious about 120fps. Played it on my X900H which supports 4k@120Hz and indeed, for the first time ever, my PS5 flashed for a second and switched to 2160p@120Hz.
Game feels very smooth, though the hit in image quality is noticeable. Not just resolution; draw distances, pop in, foliage, maybe even texture resolution are noticeably reduced. Switching back and forth between 60 and 120fps is a
very dramatic difference in terms of IQ. 60fps actually feels choppy for a second when going back and that's a sentence I'd never thought I'd write.
The game is locked at 120fps, BTW. You can actually enable an fps counter and I didn't see it drop once.
Will 120 fps help responsiveness even if your tv doesn't have 120 fps?
I don't think you'll get the toggle to enable 120fps unless the game detects a 120Hz display, at least on PS5. (I could be wrong).