Sprocket set have two setups one for acceleration and one for top speed and there is a limit to which you can go 4 any of them.
Less teeth at front or more at rear means more acceleration.
More teeth at front or less at rear means more top speed.
Keep in mind, increase is one thing will decrease the other like go 4 top speed and you will lose acceleration.
About fuel average, Increasing top speed can be good for highway use but you will not get good average in city but increasing acceleration is good for city use but not for highway. *Considering that you drive on highway at 100km/h constant.
Whatever is the genuine setup go 2, 3 teeths more or less, too much variation will take all the fun away and will make ur bike weird to drive and my suggestion will be to carry on the current setup as it provides perfect balance of acceleration, top speed and fuel average.
About company go 4 anything Japanese.
I hope i have covered every aspect.