That explains it then. Thanks for the clarification
It's on directfb. No matter how much messing around I tried I simply couldn't get it to render a higher resolution before applying the effects.
I've given up on SDLMAME on directfb for now on a cab. It works but it's either ugly, or slow. And the lack of a reasonable frontend (advmenu is the only decent option and that's simply too far out of date) is a downer
The machine is currently being updated to lenny. I'll probably just throw X + WINE + baseline on, since performance under WINE is absolutely fantastic