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Shot peening works by introducing a "compressed" layer of material at the surface of the component, the process is used extensively in spring manufacture for increasing fatigure life.
Fatigue failures occur in instances of cyclic tensile stress, hence by introducing a compressive stress layer the net tensile stress is reduced and fatigue life increased.
I can't see how this would improve push rod stiffness but it would still prolong component life because push rods are subject to bending stress (read tensile stress at component surface) during operation due to load eccentricities..
One critical point shot peening uses spherical shot and does not introduce any sharp indentation which drastically reduce component life, shot blasting is not so controlled and can be detrimental, it is also very important to ensure complete coverage of the component as failure to do so can also lead to shortened component life.
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