As of BMS 4.36 (and Win10 21H2) the defaults are all pretty good.. both in-game, NVidia console, and OS settings.
//TODO: AMD and Intel graphics?
In borderless-window mode, BMS even seems to support the Windows 10 "fullscreen optimizations" well (as of 4.35.x and later).
Here are my more detailed notes..
OS settings:
For Win10 21H2, I keep Game Mode=On and I add 'Falcon BMS.exe' to the list of desktop apps to prefer high-performance mode, although I'm not sure the latter really does anything.
For NVidia 10-series and later, you can turn on HAGS (hardware-accel GPU scheduling) on the "Graphics Settings" page, which will reduce CPU load from DWM.exe but maybe add a little (?) GPU workload. So it's a ymmv thing. For any CPU with 6 or fewer cores, I'd probably recommend it -- unless you're maxing out your GPU (keep an eye on TaskMgr).
NVidia console settings:
These settings are somewhat confusing .. if you run borderless mode (recommended/default) many of the selections on the NVidia console (and the BMS graphics page, see below) simply won't apply.
I do like to set "perform scaling on GPU" (on the desktop-size-and-position page) which forces Image Scaling=On (update: this appears to be no longer true, with latest NVidia drivers). This prevents the PC from ever trying to switch the monitor to different resolution or refresh-rate.
On the 3D settings page.. in theory, I like Low Latency Mode (LLM=On) but in practice I haven't measured it to have any effect with BMS.
I do recommend setting a Max Frame Rate limit, if you run with triple-buffering enabled.. But this also really depends on what frame-rate you're trying to achieve, and g-sync and v-sync considerations.
For everything else, the defaults seem ok.
BMS in-game Graphics settings:
These settings are also confusing .. because, again, if you run borderless mode (recommended/default) some of the selections simply won't apply -- eg. the 3D view will always be the native resolution and refresh-rate of the selected monitor on your Windows desktop.
I recommend cranking up the anti-aliasing to smooth the jaggies on the side of the HUD. (I like 4x for 4k displays.. 8x for anything less than 4k. but individual taste may vary. Update: the shader-based FXAA seems to have improved recently, and taxes the GPU far less than traditional AA.)
I also recommend turning on Triple-buffering on this screen, which keeps the rendering pipeline unblocked. But then you may wish to set a Max Frame Rate limit on the Nvidia console, per above - else your PC will get very warm and waste a lot of energy, especially if your monitor is limited to 60hz.
BMS Config file settings:
Shadows-on-smoke is off by default (good) but ordinary shadows are on.. this has a pretty big CPU hit (but it looks great in the cockpit, on a sunny day or even a moonlit night, so it's definitely the biggest tradeoff).
The various glass/water envirnoment mappings are all on by default .. they don't add a lot of value, to me, but they don't seem to cost much either.
The various shader fx (heat haze, rain drops) can vary greatly in cost, depending on your GPU and monitor resolution.