That old lotus comparison is great but it always confused me a little, because it doesn't change just 1 channel at a time. So I've tried to put together a new one, hopefully I've got this right.
The 3 channels correspond to (I think!) this:
Red - specular intensity
Green - reflection sharpness
Blue - reflection intensity
So as such, a RGB map with maximum values (white) produces what you would expect, bright specular highlights, sharp and strong reflections:
Next up, 0 red. Looks very similar, just less specular highlights. (Also don't look at the bumpers in these comparisons, it does not change here):
0 Green is much more extreme of an effect, with no sharp reflections, or in other words extremely soft reflection. I'm not 100% sure why it looks as it does, my theory is it diffuses the white parts of the sky so much the whole car is near white (and shows off the spec on the detail texture):
Then we have 0 Blue - this time it pretty much removes the reflections completely, giving an extreme matte effect. A mix of blue and green works well for a better looking matte effect:
But in practise, I would never suggest using a plain RGB map, I would always combine it with the AO bake to try and make it appear more natural and varied.
So I use this in 90% of cases, Maximum Green which gives sharp reflections everywhere, though the red and blue channel varies from 0 in shadowed areas (lower half of bodywork for example), which helps stop the bright reflections and highlights on the bodywork from the road:
Here is a final comparison of that effect in-game (not my car) -
RGB Map based on AO like shown above:
Pure white RGB map, so max reflections and specular everywhere:
Maybe not an answer to anyone's questions but hopefully useful to someone.
Edit*
Final thing - for most cases its fine to have your main skin RGB map as an uncompressed (no alpha needed, so 888) at 512px. I'd only bother using a higher resolution RGB map if its for special skins with more than 1 effect needed, so say a livery with gloss and matte elements.