Ahh, I see! This is a common problem with things that are "known" to be true -- often people don't rigourously test them.

This paper [1] has some good data in it:

"The experiments in this study were developed to verify the influence of the grain-flow orientation on fatigue life and its impact on the anisotropic properties of a mechanical component. To this end, steel specimens were made, and their fiber was oriented by machining and hot forging. Subsequently, they were subjected to flexo-rotational fatigue tests in a piece of specific equipment to determine their fatigue life."

(...) They then describe three parts: A, properly forged, B, improperly forged, and C, machined. (...)

"The results showed that specimens of configuration A achieved a much longer fatigue life than configurations B and C, actually doubling it. The results indicated a similar fatigue life behavior between configurations B and C. It is important to emphasize that this similar behavior between these two configurations is valid for this case analyzed (...)"

[1] https://www.mdpi.com/2075-4701/13/2/187