Great taste in bikes, I've had a naked 1999 GSX750 as well
(AFAIK they were only made from 1998 to 200? , 2002 - 2003 maybe ? )
I never did better than 5.2 l/100km - some 45 mpg - and usually 6l/100km though I wasn't really trying back then.
The better you streamline the GN, the faster it'll go on the same power.
But if you want the best FE, put your usual speed at the rpm where the engine feels most happy, or where its BSFC is if you can find a chart for it.
It'll still go fast for a GN250.
Taller wheels will very likely bring more spinning mass, which is counter productive to reducing fuel consumption.
If it's the overall gearing you want to extend, I'd go for an even smaller sprocket on the rear.
For tyre choice, 17" wheels would open up a lot of options, but the usual crossbrace on the swingarm might get in the way.