It should apply to all those consoles because they pretty much have the same timings: 341 total/256 active for (S)NES and 342/256 for Genesis. I don’t know where the calculation came from but the 8:7 ratio is not related to the 8:7 ratio of 256×224. Actually NES and SNES are also 256×240 but SNES just outputs 8 black lines at the top and bottom to have some more time in Vblank so it can load more data into the PPU.
I have no idea about the Genesis 320 mode. If you would capture at line2x generic 4:3 and resize it to 640×480 then the width of the image without the left and right borders should be close to the correct resolution.