No, it’s not normal to expect a drop in MPGs. It’s possible it has to do with the install even if unlikely. IMO, if you take it back with the complaint that it runs better but mpgs are worse, you’re not going to get a very sympathetic ear. If you take it somewhere else you would get a better inspection/diagnosis but it’ll cost you. Are you mechanically inclined enough to do a visual of the vacuum lines? The newer 5th gens with the air pump have plumbing that must be removed and replaced, preferably with new gaskets. Did they charge you for these gaskets? Did you verify the plugs installed, or on your invoice are correct for your year?