After much reading performance tips and fucking around on my mid range PC I have the game running with dips no lower than 40fps in New Atlantis MAST area (most demanding area of the game I've found ). This means gsync works fine so game is smooth and very playable and looks amazing.
Specs
3070 ASUS OC undervolted to 0.900 with 1000Mhz boost to memory
Intel 12400F (6 P-cores)
16GB RAM
DLSS mod with 75 upscaling from 1080p to 1440p
Enabled resizable BAR using nvidia inspector
ReBar feature ENABLED
ReBar options 0x00000001 (Battlefield V, Returnal, Assassin's Creed Valhalla....)
ReBar size limit 0x0000000040000000 (Battlefield V, F1 2022, F1 2021, Assassin's Creed Valhalla...)
Used alternative graphics preset from this mod - using the Ultra preset, still looks amazing.
STARFIELD performance BOOST!Perfect for DLSS and FSR!
www.nexusmods.com
Nvidia control panel settings
16x AF (need to delete cache in appdata/local/startfield when switching)
Prefer maximum performance
Antialiasing gamma correction off
Anisotropic sample optimisation on
Negative LOD bias allowed
Used process lasso to turn off hyperthreading for starfield.exe, also excluded from Pro Balance and enabled bitsum high performance mode
Windows graphics settings high performance.
Only thing I've seen occasionally is that the game freezes for few seconds but this happens seldom enough that it's not an issue for now.