Any danger flying the Mavic with my iPhone battery set to "Low Power Mode"?  I find the DJI GO APp sucks the life out of my battery and doesn't allow for much flying time.

I think the "Low Power Mode" will not affect your flight.

Do you have the "cash video" option enabled in the Go app?
That drains a lot of power frome youre iphone, and put the phone in airplane mode.

Go flying with your battery fully charged. You'll get around 2 hours flight time out of a fully charged iPhone battery if not more. I'll go out on a limb here and say you don't have 6 or more flight batteries.