First I would verify that your stiffness is coming from the controls themselves or from the cables, linkages or outdrive shift cable. Is it stiff when you are using the controls to apply throttle only (warm up mode where you press a button or pull the lever out to disengage the shifting) or while shifting and applying throttle? You can disconnect the cables at each end to track down where your friction is coming from.
In many cases the shift assembly feels stiff because the outdrive shifter cable needs to be replaced. This cable runs between the control connections near the top of the engine, through gimbal housing to the outdrive bell housing. There is a piece of hardware attaches to the inner cable and when it accumulates stiff grease, corrosion gunk it screws up the shifting. In my experience it should be changed out about every other (or every) bellows replacement.
