I have a K40 with both the Harrt's rod and a 22# spring -- of the two, I think the increased spring weight reduces felt recoil more. I can't honestly say the Harrt's rod makes a huge difference, but probably helps some with muzzle flip and follow-up shot placement. I would try the 22# spring, which is much cheaper, especially if you notice that your brass is travelling excessively far with the light spring, and don't forget to retest it for function with your carry ammo. If you still want more improvement, shoot another case of ammo, and then if you're still not happy, spend the $75 on the Harrt's rod! Best wishes.
Andrew