Reset All Settings didn't help with my iPhone 7 Plus when we tried it at the Apple service center.
But there is some possibly encouraging news. My persistence in sending emails to various people at Apple, including my side-by-side photos might be paying off.
Yesterday I received a phone call from an Apple executive office and they agreed with me that my iPhone 7 Plus display did not look as advertised, compared with my iPhone 6 Plus and they are escalating this with their engineering team and said they would call me again today.
So at least Apple is not ignoring this now and maybe something will happen.
Other people have reported that after multiple exchanges they did finally end up with as-advertised brighter iPhones.
And I've read some technical articles about how there is some manual per-device visual backlight LED calibration that takes place during manufacture to achieve a certain color gamut (which, if true, is astonishing).
So it does seem likely that there are per-device variations in display quality.
Anyway, at least they are finally listening to me about it.
I hope I have better news to report soon.