I don't like the "double everything to 512" approach...unless proven otherwise, in the end Seiken Densetsu 3 is running with HRES=256 and the other test cases runs at HRES=512, so the CRTC is presumably doing some post-processing with the internal framebuffer and trigger the internal 256/512 switch caused by modes 5/6 at vblank time. It works for now for either MESS and for BSNES, but imho it's definitely something that requires tests on real HW if possible (for example by quickly switching between mode 1/5 on a single screen and see if there's or not a noticeable flicker on the resulting screen).