RE: ninja 250 sprocket
You can figure 5mph more and 500rpm less if you go to the 15 frount sprocket. its the cheapest one and easest to do, plus it makes your chains last longer. If you go 42 in the rear, its another 5mph increase plus another 500 rpm's less. So it is almost like adding 7th gear.
Yes there is a small power loss in low end but I am not a dragster, for that I will use the ZX14.