The founder of Go Power did this work while working on his PhD at Stanford
about 60 years ago. He was an avid go-carter and the emphasis in those days
was "port and polish" . . . with emphasis based on "polish". He debunked
that myth in his thesis.
You are correct in that here is a boundary layer. That layer also has
"static cling" LOL, or more technically "stiction", capillary attraction, or
call what you want, with the passage wall. The thickness of this dead-zone
layer is proportional to velocity. But, regardless of velocity, the flow
attached to the wall is essentially zero. So any protrusion in that area is
irrelevant. Only discontinuities sticking up through the boundary layer
influences flow. IIRC, anything with 60 grit (about 1/16") is fine. As Dave
has already indicated. I think some studies showed even a 120RMS had little
influence on flow.
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