Just wanted to add my thoughts on this problem.
I picked up a 7 Plus in September and returned it 3 days later. I must have had a dud unit as the images produced by it were awful. I returned it as I just did not want to go through the process of proving it to Apple for a warranty claim.
Here is a small album of photos compared to my Nexus 6P
https://imgur.com/a/f3OSq
The photo of the Guinea Pig is the wrong colour, the hay and cage is the wrong colour, his fur is blurry and blotchy. The image is full of digital noise.
Same for the photo of the Ford engine, wrong colour and blurry in parts. Whole image looks soft.
Disregarding DCI P3 / SRGB, the images produced by the 7 Plus are worse than would could be produced by my old iPhone 6 Plus with its 8MP sensor.
It's not a huge issue for me anymore as I pitched the phone but wanted to confirm for others that yes this problem exists.