That's a pretty big gearing change right off the bat. If you don't have too many miles on the sprocket/chain combo (you should always change chain and sprockets at the same time-so they say), you could change the counter shaft sprocket first and see how it works. If it's still useable then change the rear sprocket (you may only want to go down a tooth or two).
If your bike is like my Versys the speedo/ odometer will be off (signal comes from the transmission not the front wheel). You'll have to figure that for your speed and mileage.
I think 80 mpg is very doable
, Kawaski rated the 2009 ninja 250 @ 61mpg.