Kindly confirm the optimal sprocket replacement options to improve performance on the KLR650?
"Optimum" in this context appears subjective, to me: Depends upon your riding style and environment.
And . . . a gear change won't "improve" performance, per se
. Rather . . . changing the ratio changes the torque multiplication of the drive train, not unlike shifting gears. For example, changing from second to third gear does not "improve" performance, but . . . second gear multiplies torque at a greater ratio than third (and higher) gears.
Heavily-laden bike, ridden slowly over rugged off-road terrain, maybe you want more drivetrain torque multiplication: Dropping down a countershaft sprocket tooth, or adding a few rear sprocket teeth might be more, "optimum" for that situation.
Whizzing down the Interstate unladen: Maybe you want greater speed vs. engine rpm; you may want more countershaft sprocket teeth, and/or fewer rear sprocket teeth.
The AMOUNT of torque multiplication is a direct and simple mathematical comparison: For example, changing from the stock 15-tooth countershaft sprocket to a 16-tooth countershaft sprocket reduces torque multiplication by the factor of 15/16. Going from 15 teeth to 14 teeth, torque is multiplied by a factor of 15/14.
Please keep in mind: The absolute torque produced by the engine remains unchanged; changing sprocket teeth number affects the multiplication of that torque where the rubber meets the road.