Hi Toni,
today I've experimented with NVidia G-Sync + Fast Sync and noticed it's actually better then leaving "Use the 3D application setting" for vsync: WB refresh (emulated Picasso Z3 1080p on a 75Hz display) looks a bit faster/smoother e.g. when dragging solid windows, and in-game input emu lag (almost absent IMHO also with standard G-Sync mode. I personally never perceived lag issues playing platforms and shoot'em ups using g-sync) seems also slightly improved…
Does it make sense?