I'll break it down for you in one sentence.

If you're this concerned about MPG and you need to calculate the savings/loss, don't get it, the remap is NOT for you.

 

The BMW TIS says that most "chip tuning" gains power simply by injecting more fuel. I guess that's only when you want it i.e. when you put your foot down, but means you'll get more power at the expense of MPG and at other times the MPG will stay the same.