If you install the ones by default, you basically don't need to do anything. GM will take care of the blank space so there's no degradation of any sort. You could create extra super resolutions for special cases but I don't think there is anything except maybe the Sinclair Spectrum that is not covered properly by the default list.
The problem you're facing with vertical size is the same that the other folks have explained to you in your other thread. You can't fix that by adjusting things in Arcade OSD, etc. It's a physical feature of CRTs.
What you can do is to create a custom crt_range changing the progressive and interlace line limits, e.g. change this:
crt_range0 15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 288, 448, 576
to this:
crt_range0 15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 256, 448, 512
or:
crt_range0 15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 240, 448, 480
This will force those vertical games to be displayed in interlaced modes that will probably fit your screen, at the cost of some flicker.