Well, mine is a high top 'glass topper. I don't think the weight had anything to do with the MPG hit, it was all in the aero drag of the high top design.
Between the topper removal, and avoiding E10, I've picked up close to 5-6 MPG overall. A very noticable improvement when gas was hovering in the $4/gallon area.
I like the extra storage area, but it's not something I need, most of the time.