Editing iPhone 7 Photos on sRGB iMac Photos taken on an iPhone 7 are encoded with the DCI-P3 color profile. Would editing those photos in the Photos app on a 2013 (non-Retina) iMac result in loss of quality?
From doing my own testing, it does appear to be the case. I edited a photo on my iPhone 7+ to soften some highlights. When viewed on my iMac, the highlights were still blown out. I further lowered the highlights on my iMac. Now the photo viewed on my iPhone had blown out highlights once again. Trying to edit it on my iPhone showed all the color controls at 0, even though it was clear it had been edited.