I have set up Scart out on my Philips 28" TV with Pi2Scart and this is a tricky one to call. The interface worked perfectly, although the Pi can no longer be seated in the Picade enclosure (in fact it will require some work just to get the door to close).
The default parameters worked perfectly;
disable_audio_dither=1 dtparam=audio=on dtoverlay=vga666 enable_dpi_lcd=1 display_default_lcd=1 dpi_group=2 dpi_mode=87 hdmi_timings=320 1 16 30 34 240 1 2 3 22 0 0 0 60 0 6400000 1
And I played around with some optimal output resolutions from pi2jamma
but in the end I got best results from letting libretro output the native resolution with some black bars on most games.
The picture is now a lot more stable (you can see some of the flicker in the composite screenshot) and the colours are more vibrant. However games higher than 240p look clipped and the Emulation Station interface is tricky to use at the lower resolution.
The best of both worlds would probably involve having a separate Pi+Pi2Scart for Mame games up to the 1990s. Anything after Neo Geo would probably be better, even with flickery composite.
You can open the images in a new tab if you want to zoom in. The pictures were taken with auto focus and exposure, so they're not 100% accurate.