On the weight thing, absolutely. Same principle behind the fairings. Less drag, less pull = freer movement = better mileage.
Weight and mileage were actually why I chose getting my 40v moby over the 50v moby the seller had available (it was also cheaper and in better condition, honestly, but i did my homework). I can trim a lot off the 40's frame that i can't off the 50's, bringing the frame down to probably 15-20lbs. lighter than the 50's, giving me that much more MPG room to wiggle with larger engine bits if I wanted to.
Nothing impressive about a ripper that needs re-filling every 45 miles.