One number for gas mileage is pretty meaningless. The type of driving you do is the primary variable for mileage, and that can't be accounted for in a one-number mileage poll. I get about 21 in surburban driving and 28-29 for freeway cruising.
This mod is just what I've wanted since the first time I drove the car at night. But... I'm dangerous with tools in my hand and would certainly screw this up or get it half way and not be able to finish. Are there people/services/businesses that are good at things like this?
P.S. No thanks to Subaru for leaving the front compartment dark (really dark in the wagon).