I just got a silver 13" tbMBP from Adorama (it's a BTO spec that they had in stock - 3.3GHz i7 / 16GB RAM / 512GB SSD) and it unfortunately has the slight darkening on the bottom third of the screen. The color uniformity is really good, but there is darkening at the bottom (that can appear to be "yellowing") that I didn't notice right away. It definitely varies in intensity depending on the angle of the display. I took a picture with my iPhone but it makes the problem look worse than it actually is.
I'm disappointed, especially since my 2012 rMBP and 2015 rMB exhibited better uniformity, but
anecdotal evidence seems to indicate that this is the nature of the beast with these P3 screens. I dealt with a couple generations of iMacs that also had uniformity issues until I got fed up enough to get a nMP and brought-my-own monitor.
Most of the pics posted here look more or less like my display, and since I bought my computer through Adorama, I can't do the exchange game and would instead have to rely on AppleCare which I'm not inclined to do.
Oh well...can't really say I'm surprised. My computer is perfect in every other way (NO scratches or marks - and I inspected closely), crashing, or other weird behavior, so I'll live with it.