The MPG difference should be minuscule, but you might find that you notice the extra weight in accelerating and braking. My car has weighs less than yours, but I really feel it when I have a passenger.
My vote would be replacing your tires, even if it's only the rears. Not all tires are created equal when it comes to noise transmission, and my (uneducated) guess is that you're not hearing noise being transmitted through panels, but rather, it's coming from the suspension up into the frame. Dampening your floors will probably do little to help.
A quick google search returned this:
DIY solution to deadening tire noise
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