Hi,
hope my answer won't be confusing, I have bit mushy brain from lot of work late few days. So my answer comes in form of notes :- )
- Tough GI is.. bit problem in Corona, well in most rendering software. No way around it... to clean it up, some scenario suddenly require 10 times the render time more run of the mill scenario would.
- I run into it quite often. In theory things like "Light Portals" should help, and they sometimes do...but results wary and I was never quite impressed by it.
- Possible solution is to simplify the lighting scheme but achieve the same thing. In the Impremta Bathroom, there is no Sky/Sun/HDRi outside, there is square area light. The result is visually identical, because that's what small portals do to light in reality. It renders much faster.
-Materials with broad specular/low glossiness like satin paint, with micro structure....are hell. They never... never clean-up. What I do...is I wait forever...and then use bit of denoiser on top. Not too much, to avoid the painterly look. 0.3 +/-
- My solution was to buy a lot of computers. I am not kidding, I have 6 heavy dual-xeon machines and my render times are still hours :- ). In fact, every year I have more computer power...and my render times always increase :- ).
- It doesn't matter if you increase exposure or light source. It's the very same thing. You avoid burnout with creative lighting, highlight compression, post-production and most importantly...accepting that burnout is natural. Burnout is your friend :- ) As long as it's clean and desaturated, burnouts happen in photography and bring that life and dynamic into image.
- Yup, the hidden noise in shadows is very similar to photography. It is also intentional behavior due to adaptivity. Denoising might be the only solution here.