My Kawasaki Zephyr 750 gets about 53mpg normally and sounds lovely doing so. Mind you, I never really rode the bike trying for economy... too much fun to be had.
I have also commuted a few times with my 92 Honda XR250L and the best I saw was about 78mpg.
I no longer commute on my 250 because I felt like I was over stressing the motor cruising at 90 or 100km/h when it tops out at about 120km/h.
I say just enjoy the bike, chances are if you go down to a 250 or so you'll end up missing the grunt of the 650.
BTW:To be on the safe side, I'd go back to the recommended tire pressures on the bike.
My old bike bothers me, for being so small, and light (400cc, 400lbs), it gets horrible gas mileage! only 30-40 mph. I'm thinking something is wrong in the 30 year old carbs. Power is still good, 100 mph in tight pants, and t-shirt, 95 in a sweathshirt, 95 in a coat too, but takes longer to get there, probobly had more power from the cold air too.
I've always been a fan of good aerodynamics, top speed seems to me to be an easier way to measure improvement or not in aerodynamics.