Here is the same scene with ONLY an infinite light at 100% - no IBL at all.

Now the non-GC looks terrible. It is too dark, and the reflected light decreases too rapidly with the changing angle of incidence.

The GC props look right.

If you look closely at the cube, the side face of the GC cube is the same color as the front face of the non-GC cube. The non-GC cube is unrealistically dark, especially on the side, but also on the front. It is not correct to say that mid tones are affected more than darker areas. In fact, if we measure the perceived error (not actual error, but our perception of it), the darker the are, the worse the problem is.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)