The X already has 17" wheels, so that's not going to be a great place for modification. I guess there are two ways to work on improving mileage, too. Highway mileage is interesting, but around town mileage would be really interesting for a bike like this. I just bought a used WR250X and it seems like the perfect urban commuter if mileage could be improved. For the last few years, I've commuted on a Kawasaki KL250 Super Sherpa, but that bike is all but worthless in the cold months because the carb just refuses to work in cold weather. The injection on the WR is the cure for cold weather starting.
Tuners seem to be clueless about improving efficiency performance. I also ride a 650 V-Strom and I've never found a tuner who knows where to begin in improving that bike's mileage. They have all sorts of ideas for getting more power, but mileage is completely beyond their scope.
Tom