ok, some of what you're saying just isn't making sense to me.  5% ambient light should, i would think, mean that if it's lit by nothing else, it should be at 5% lumniance, right?.  and all the way up to 100.  nothing should make 5% ambient light translate to say 20 or 50 % luminance, right?

actually, i was mainly going by the atmosphere, but if you think the atmosphere shader needs to be GC corrected, i guess i could see that.

just going by eyedropper, my original gave me colors with a luminance of about 5% when lit by the IBL and infinite in an area only lit by the IBL and the infinite.  i could see where that's too dark.  VSS is giving me colors with a  luminance of  about 23% in the same area.  with just an infinite at 2% to take the shadows away from dead black, i'm seeing colors with a luminance of 15%.

i would think the proper situation would be that if you took a figure and put them in a scene that matched exactly an HDRI or IBL source, and lit them with an HDRI using that source at 100%, you should get exactly the same lighting.  if it has to be at 20%, that seems to me that you're subtracting from the original amount of light.  if not, then why?

that doesn't sound even vaguely correct to me, but i openly admit i'm not an expert like you are.

and i would think that it would be constant, and not respond differently to ambient light than, say, a spot.  as i mentioned, i'm not seeing at all a simple too bright figure.  i'm seeing a too evenly lit figure.  

i've collected a bunch of low light images, i'll add yours to the collection.  and i'll keep testing. but so far i'm still not able to get anywhere near the results i'm trying to achieve or even that i've seen in dozens of photos.

i will say this puzzles me:   shifting the gamma on photos doesn't make them not look like photos anymore.  heck, doing crazy stuff with levels, curves, and blending layers doesn't  generally make it stop looking like a photo.  so i don't see why something without a gamma shift in rendering would make the main difference between photographic or not?  also, i see yellow bloom in lots of photos where the light is too bright or too close to the subject and the skin gets overexposed.  so why shouldn't i see it in renders in the same conditions?