What camera app were you using in the iPhone 7 Plus?
Apple's default Camera app? FiLMiC Pro? (That makes a world of difference to your ability to calibrate exposure. And were you shooting Log with the iPhone?
What frame rates were you shooting at? Were both cameras at 24fps?
Was the ARRI shooting ARRI-Raw?....
So curious about a lot of this....

Hi Justin.
You're going to hate me.

You wrote "...(his second, the first coming from his role in Lucas's The Last Picture Show.)..."

The Last Picture Show was directed by Peter Bogdanovich.

(**I'll set my head on the chopping block. Make it quick.**)