The change in drive ratios from changing countershaft sprockets is linear and easily calculated:
With a 16-tooth countershaft sprocket, you get 15/16 the rpm at the same ground speed; 15/16 the torque multiplication at stock rpm.
With a 14-tooth countershaft sprocket, you get 15/14 the rpm at the same ground speed; 15/14 the torque multiplication at stock rpm.
Example: Stock gearing, 5000 rpm. With 16-tooth sprocket, 4688 rpm; with 14-tooth sprocket, 5357 rpm.
Lets say you're at 20 ft-lb rear wheel torque; with a 16-tooth sprocket you'll get 18.75 ft-lb; with a 14-tooth sprocket, you'll get 21.4 ft-lb torque.
Deltas (changes) worth changing sprockets? Only your own riding style and environment applies!
FULL DISCLOSURE: 15-tooth KLR650 countershaft sprocket hasn't failed me; I have a 14-tooth on my KLR250; works great (the 250 has a six-speed transmission).