Check out my 101.5mpg Ninja 250 thread. Just gearing mods and riding technique got me there.
http://ecomodder.com/forum/showthrea...50-a-8686.html
I went with a 15/39 setup for a 2.60 final drive ratio. In my experience, anything lower would be detrimental. My bike already slows down considerably on minor upgrades in 6th gear. With a lower final drive you certainly can use 5th gear on the highway, and maybe leave the availability of having an extra low 6th gear to gauge future aero mods. However, i don't think all the aero mods in the world will allow a really low 6th gear to be functional. Hope you prove that wrong!
You can not fit larger than a 15 tooth front sprocket without both removing the sprocket cover and grinding away at the case.
I'd also advise away from attaching a fairing to the front wheel. You'd be much better off extending the stock fairing out over the front wheel.