OpenGL 1.3 and 2.0: performance improvements but what are the differences?

vid_gl13 0 (the OpenGL 1.1 path) does virtually nothing, but should usually be a bit slower. It's mainly for tracking down driver bugs.

vid_gl20 0 (the OpenGL 1.3 path) however is a large change, but lacks some features.

As for "It does however raise the question if low and medium settings are compatible with the OpenGL 1.3 path due to the missing reflections.": maps that rely on the reflection effect for gameplay force the effect on by map-specific configuration. So the low config is even fully playable on these.