There's no real vbios. There's a CGA chip (6845), with pretty limited registers to adjust timings (and a lot of software out there that expects to set these directly or for line timings and frame timings to be precisely known values). The clock rate is fixed.
The scan converter in panels often guesses "wrong" about modes. If one could change the firmware for it, that would help.
I'm not sure the BIOS can help much.