Been doing some more low-res experimentation. So, I've found a few things out. One, all in-game videos (video at start of game, etc.) will look like garbage and do weird things. They won't crash the game, mind you, but they will look bad (this is to be expected, those videos have a specific width and height they want to be at, I suspect, and being forced into something less than that causes weird things - though it is kind of interesting to watch this game's narration over what looks like an M. Night Shymalan fever dream.)
The character creation screen is kinda borked, not sure why exactly. Cuts off certain models. Doesn't really affect the game much, only cuts off the bottom part of the model (just in the creation screen, the character looks normal in-game).
The game will definitely run best if you put your display resolution to what your screen's optimal resolution is (via the in-game screen resolution). I had some crashing problems when leaving the game and going to the start screen again, but when I changed the display resolution to match that of my monitor's resolution, everything seemed to work out fine (did it 3 or 4 times to make sure).
So, out of the major issues you might have when lowering resolution to increase performance, the video screwing up is particularly jarring. Still, considering how few and far between video is, I wouldn't say it's a dealbreaker. At some point I suppose a switch could be made to render the videos at a proper resolution when in a low-resolution mode, but it's not honestly super-necessary (if you're willing to play it at low resolution, I suspect video quality is not going to hamper your desire).
