I’m glad I stumbled across this article! I’ve recently moved to saving my photos for IG out of Lightroom with the P3 profile. It’s been great to see my images in a wider gamut even on the iPhone 6p. The issue I’m running into is even when using the P3 profile, IG is serving back to me a much more dull image. Am I being served back sRGB? I wanted to make sure you were aware there are color gamut gains available with P3 even for pre-iPhone 7 devices.