"How much it will help" depends on lots of things. On my '00 Grand Voyager removing it seemed to be a dependable 1 mpg gain. Usual tanks (no ecodriving) were 18-20, without the rack they're 19-21. YMMV
As for how to remove it, mine had five screws on each side -- two in the front and back, one in the middle. The screw heads were "hidden" in the mounts a bit, but they came right out and the rack lifted off. I put the screws back in the holes to kinda' sorta' seal them up, also so I wouldn't lose them when we sell the van.