Tim, grease on the hammer will definately slow it down, however I would not reccomend it as a "tuning"method. What will happen is as the grease travels in the hammer area, it will change the speed the hammer hits the valve, this will cause inconsistent pellet speed. You are better off using only a light lube on the hammer and tuning the rifle by changing the spring or simply cutting the one you have.
Hope this helps
Chris