Jack, I'm not trying to hijack your train of thought, but I am a firm believer that there should be an upper intensity limit based on media size. In my experience, damage is induced well before small media buries itself into the workpiece.
I know of multiple tempered martensite steel products in the spring industry where small shot at high velocities produced a subsurface defect whose presence correlated very well with significant reductions in fatigue life.
Modifying the process to use larger shot at lower velocities to produce the same intensity eliminated the subsurface defects and greatly improved fatigue life. (In one case, the fatigue life more than tripled.)
So, here is a situation where there is no detectable surface damage but fatigue life is greatly impacted by the details of the peening process.