I use G-Sync in P3DV5, and it can go right down to around 20fps and all the way up to 60-70fps (in p3d, more in other games) and still feel good. It’s night and day over vsync or half refresh rate or any other concocted things people do. I personally wouldn’t ever fly without G-Sync activated again. Same applies to XP-11 with G-sync too.
ps, G-sync works from 1hz right up to monitor limit (100,120,144,160,175hz etc etc). But I believe freesync has a lower limit of 30-40 frames depending on the monitor in question, and below these frames it uses LFC. This is because freesync is driver based, but g-sync is actual hardware inside the monitor. (Hence the higher cost!)
PS here is a list of all the variable refresh rates of each gsync or g-sync compatible monitor. https://www.nvidia.com/en-us/geforce/products/g-sync-monitors/specs/
PPS. For a sim as demanding as P3D, where frames can drop to 25-30, I’d recommend a gsync monitor over a freesync one, purley because below 30 frames g-sync is far superior. A VRR of 1-100+ plus would be my recommendation.