I don't even understand this question. You started out by asking about Amiga output when run through a SD/FF. If you're running native Amiga video output through a SD/FF, for example an Indivision ECS, it's going to output all modes at 31KHz. Different SD/FF's may offer different capabilities.
If you instead hook up a graphics card like a CV64, PIV, etc., you're going to get the benefits of whatever graphics mode you use, which is completely variable depending on the modes and capabilities of that particular graphics card.
A floppy-booted game is not going to take advantage of any of the capabilities of a graphics card. So if you want to promote a game to run on a display device that doesn't sync to native 15KHz Amiga output, you're going to need to use a SD/FF (i.e., Indivision, GBS-8220, DCE ScanMagic, AGA-2000, A2320, etc.).