"I guess exposures are expected to be slightly less accurate with a limited scene detection system. Is this assumption correct?"
Yes you are correct. Compared to my D7000, my D800 always hits exposure. The D7000 would miss often. Not often enough to make the 7000 a bad camera, just not a pro camera. But I was always able to correct the miss in post. Sometimes the 7000 would miss bad when using the flash (on or shoe). Those could only be corrected in post if I had the raw.
Not sure what roll the expeed III has in the metering. But Im sure the 600 will do better than the 7000. I would not worry one bit about the RGB sensor. Impact will be small. Impact will be 0 if you shoot in raw.