Based on what I've seen, read and my own tests - it seems like the bug is an exaggeration or over-/re- transmitting of the data that's actually getting transmitted. Case in point:
On Monday, I sent an email with a photo I took. The photo was 2.7MB. To check it, I clocked my data before and after. I consumed just over 4MB to send a 2.7MB email.
Clearing all running apps out, if I open a few apps (none of which require anything more than checking in to a back-end), which on Android only consumes about 20-40kb, I blow through 500kb.
If I use Shazaam to identify a song - something that takes 20-30kb or so on Android, I blow through 500kb on iPhone.
I remember running Speed Test the first night I got the iPhone (this is my first LTE experience, wanted to clock it!); running Speed Test one time blew through 30MB of data! Now I know that Speed Test doesn't transmit that much data to run a speed check.
For the past few days, I've not used the phone except for light email checking and a few apps opening. Based on the past two years on Android - I should be consuming maybe 500kb or even 1MB. My average daily consumption over the past few days has been between 20-25MB.
Tonight is my last chance to take the iPhone back (I'm at the end of my first 14-days) and I am seriously considering that. If I knew the problem was being fixed, I could wait it out, but not knowing if Apple is going to fix this, since they're all quiet -- I can't take the risk of keeping it for two years.
Very very frustrating!