There shouldn't be much tension on the shift cable. Look how skinny the cable is to begin with. It's at least half the diameter of the brake cables. And the lever on the shifter is only an inch or so long. Not a lot of force is used there. The clutch spring in the hub is something like 1lb or less. It's just a big diameter pen-click spring. If there is more than 5 lbs of pressure on that shift cable when it is properly adjusted and maintained I'd be really surprised.