Command line I'm using is simply "sdlmame (gamename)" and I'm setting the shader in mame.ini via "bgfx_screen_chains hq4x".
Here's what the shaders look like in MAME - http://imgur.com/a/Xu1w1 - you probably don't need the RetroArch ones for comparison as this isn't a case of them looks "just slightly off" haha.