Anyway, it's looking like to be safe in fullscreen mode you're going to have to make custom sizing and black-bar-cropping stuff no matter what.
Well, I think the stretching of a 4:3 ratio game is minimal when displayed on both 6:9 and 6:10 ratio screens (no idea what it looks like on a 5:4 ), so I think I'll go for 4:3 at 1024x768 for anything I create.
I've uploaded the results of my experiments the last day or so in the uploads section:
It runs in a borderless window —, and stretches to fullscreen without changing the resolution with no impact on performance, and you're just getting the graphics card to do the scaling rather than letting the monitor do it when you change resolution.
I don't know what the OP got out of this thread, but at least it made me tackle something I hadn't got around to looking at.