EDIT: Sorry guys, I drew too quick of a conclusion because PUBG and FOrtnite shared the same issue on the same engine as Valorant.
The issue appears to be just visual / sound on clientside, firerate does not actually fluctuate.
Based on this reply https://old.reddit.com/r/VALORANT/comments/gngrna/weapon_firerate_depends_on_fps_details_in_comments/fra0brg/
I figure the only reason to cap your FPS would be to minimize such desync perhaps, assuming the calculations even apply and Valorant doesn't follow it's own rules.