First post here, hi all 
One of the more interesting, lesser known features of Groovymame is the ability to use nonstandard refresh rates even on a LCD monitor, with any video card and without having to use freesync or gsync.
The way it works on Windows is that some custom video modes are generated with the video card control panel, keeping the native LCD resolution but with custom refresh rates at 50, 54, 55, 57, 58, 59 and 61 hertz. LCDs only support integer rates.
Upon launching Mortal Kombat for example at 54.7hz Switchres engages the closest available mode on the LCD, 55hz in this case.
Since the deviation from the correct refresh is never more than half a hertz the speed is right, the scrolling is perfect and the audio stays at the right pitch. It’s the next best thing to the full Switchres on CRT setup, but immensely more practical. All the PAL roms from any system run at the correct speed, etc.
Is it possible to do the same with Retroarch somehow, on PC but especially on the Raspberry Pi? It does improve the emulation quality immensely. I know it’s possible to boot Retropie at a custom refresh rate, but it needs to be done on the fly for each different game.
Thanks, AF