well its going to weigh less than a passenger, i highly doubt you will see any mileage difference from adding a few extra lbs to the trunk.. that air pump instead of a tire is pretty ridiculous if you ask me.. i have been thinking about doing the same thing..
I dont think the difference is that big. I did a trip to Atlanta from Central Florida last year with my Cruze ECO with K&N Intake, as I did not have the tune yet. I got 45-50MPG most of the way, thats with 2 people and the car loaded down with gear and a cooler full of booze and stuff. Was atleast an additional 200lbs in the car from the stuff.