If you want any shadows whatsoever in DCS (which really increase immersion for me), you can forget about getting 120 Hz when:
- near scenery
- near other aircraft
- looking at tracers or fire
- within 20 km of moving ground units.
I struggle to get above 60 Hz and that is with a new Ryzen 5000 processor and a reference RX6800XT.
At a reasonable ~70% SteamVR res.
So I would say 120 Hz is not relevant for flight sims for the next 4 years