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Well I've been doing some homework. First I reviewed "High Tech Cycling" 2nd Ed. by Edmund Burke about rolling resistance. The advantage of a larger diameter tire is in the reduced amount of deformation that occurs rather than tire patch size.
Second I can suggest "Sport Rider" Tech Editor, Andrew Trevitt's book on chassis set up and tuning. See Amazon.com and the price is about $25 USD. Charly Perethian modified a 1989 Honda NX250 6 sp. for the 2011 Mid Ohio Vetter Challenge by switching the 19" front wheel to a 16" set up, changed the sprockets to 15/33, and added a slick front fairing to get 157 mpg. The power band is similar to the GN250. See the description on craigvetter.com.
Third, I looked for tires rated S or H speed that would fit the GN250. Check the Kenda K671 front and K657 rear for price and availability there. There are some sites that riders can rate tires for wear and traction. The OEM tire for the older Ninja 250s (16" rear) and others was a Dunlop. From the Maxxis website a 130/90 touring tire has a diameter of 25.1 in. Knowing the tire diameter, 5th gear ratio, and sprocket teeth you should be able to calculate the rear wheel speed for a given rpm. The only info on the transmission ratios for the GN250 I found was for a version sold in Portugal that has 5th listed as an 0.818 overdrive. I hope this gives you someting to go on.
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