350/175 line mode

[Wintermute68k]

The VGA standard has a 350 line mode that uses the same timings as the 400 line mode. It exists for EGA compatibility. It has the same refresh timings as the 400 line mode, but fewer active vertical pixels and inverts the vertical sync signal to tell the VGA monitor to stretch the picture vertically to fill the screen.
You can read about the timings here:
https://martin.hinner.info/vga/timing.html

What benefit is it? It's possible to slightly improve Doom performance by drawing fewer pixels to fill the screen. That's 12.5% reduction of pixels to draw, very much in the original spirit of this project.

Use the same scan doubling that turns 400 into the normal 200, but instead 350 into 175. Same idea as Mode X/Y.
Resulting resolutions of interest would be 320x175 and 160x175 (low detail). You can also make the potato version of it as well, at 80x175.