Case is closed. All timings have to be doubled for 256 color modes, cuz, I guess, symbol is 4 pixel wide instead of 8 in this mode. I.e. 640/720 modes aren't actually possible for Mode X. Actually, possible, but, I guess, at 1/2 FPS. Dunno, if real hardware will be able to handle it.
UPD: Yeah, despite of wrong window aspect ratio in DOSBox (window has 1/2 of height, cuz due to some reasons DOSBox thinks, that horizontal resolution is 2x), 640/720 modes work with double timings. 128k frame buffer doesn't work though, so any modes higher than 640x400 work incorrectly. FPS should also be 30 instead of 60. Dunno, if this program will work on real hardware. Of course I can try to install FreeDos on flash drive and try it on my current computer. But I don't have time for this now and I'm not sure, that this test would give me accurate results. I also need to change lazy per-pixel rendering to fast per-plane one.