The correct comparison would involve this, I think.

You lost saturation of the light colors, because you didn't anti-correct the light colors! In particular, if you're using an IBL image and the IBL image is a photo, then the colors from that photo have been de-saturated into sRGB space. The true color components are NOT what you're using for lighting.

When using GC, you should use linear light colors, not sRGB light colors. Hahah. So anti-GC the desired light color in the light shader, when you're using GC.

This is what I do when you make an IBL probe using my genIBL tool. That tool automatically does anti-GC when generating the probe, so you don't have to anti-GC it. But if you're using a probe made by somebody else, such as a photo of a mirror ball, that has to be brought back to linear color space. Otherwise, you're not using the correct representation of the lighting captured by the mirror ball.

This means you won't be able to compare in one render. You'll have to render GC things separately because you need the lights to be different.

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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)