I don't think it's useful to go TOO big, but yes, at least 4k and maybe experiment with 8k.
I have no UV unwrapping skillz yet, so I'm at the mercy of whoever created the AO.
I
have sometimes adjusted exposure/contrast/shadows/etc on the AO if I don't agree with how it looks, extracted via CM.
I've also found that OG modders have a huge range of personal tastes regarding shaders, and quite often restoring those back to Kunos defaults, and then tweaking from there, can give good results. And yes, certainly even for a monochome livery. Contrast/depth/shine etc.
Edit: And just to agree 100% with
@Alguecool regarding logos (not GIMP
). Don't place a logo on the car and scale it multiple times to get the size right. Each successive scaling degrades the quality of the logo (layer). I work out how big it actually has to be, have the (hi-res) logo open separately, scale it once and then place it. Same goes for transform/rotate. Do that at hi-res, scale, copy and place.
Edit Too: That prompted me to check again, on a feature coming to GIMP, called "Linked Layers". Targeted for milestone 3.2, it'll be functionally equivalent to Photoshop Smart Objects, allowing "embed" of layers, vectors maintaining their original quality.