Important to set desktop refresh rate above the emulation rate? I.e. for PAL, use at least 60 Hz desktop refresh rate, and at least 85 Hz for NTSC? To avoid g-sync driver throttling the speed? (Seems to be imported for Nvidia on Linux at least)
Having extra monitors connected is problematic. More than one monitor disables G-Sync (on Linux). Disabling the monitor temporarily in Nvidia settings causes occasional stuttering - also in window mode when G-Sync is not active (some polling of the disabled display by the Nvidia driver interfering with the output??)
Tip: Enable visual indicators and ensure that the indicators says G-SYNC in green and FLIP in green (not BLIT in red).