Hello @acris
i modified "MaintainAspectRatio" inside /recalbox/share/system/configs/pisnes/snes9x.cfg like this:
[Graphics]
MaintainAspectRatio=1 (it was =0 at the beginning)
but when i run the snes emulator, MaintainAspectRatio is restored to MaintainAspectRatio=0, and emulation stays full screen at 16/9 format
i also tried to delete the line MaintainAspectRatio=X
and it is also restored to MaintainAspectRatio=0 just after snes emulator is started
PS: i use recalbox on Raspberry pi 1 B, on TV through it's HDMI port, it works well thanks to the overclocking