I did some research and came up with a statement from an ex employee of Keith Black.
From the lips of KB "shot peeneing is good for about 2% increase in strength over non shot peened rods".
For a 2% increase my money will be spent elsewhere.

Hello, Henry.

No disagreement with KB there.

Strength isn't the purpose of peening.

Its to increase fatigue life..typically by at least double..

But, as you have pointed out, the issue with 2.2 vs 2.0 rods is not really fatigue failures...I have persinally never seen a stock 911 rod fatigue fail..tho' I know it happens..

My guess..the problem is either lube failure (most common IMHO), or maybe distortion of the cap, losing the oiling clearance, heating up, and then failure.

This I know you've seen thousands of..

Have you ever seen a definite fatigue break?
What faulure mode do you see with the 2L rods?

Kind regards
David