These are one of those situations that if you simply step back and use common sense you will see that its really not sensible that bmw M would use a restrictive elbow if there was 10hp on the table from using a slightly larger diameter pipe. The diamter being smaller is not necessarily more restrictive. Its likely calculated based on desired intake air velocity whichi requires a certain diameter. Larger diameter means slower moving air. So while it may move "more" air, the air is slower and would take more time. This intake does not have a restriction for air in totally stock form.
Evidence is in the GTS 4.4 stroked engine which uses the identical intake box as the stock s65 m3. Clearly a 4.4L if anything requires more air and the stock setup provides enough air for that so our stocker engine has more than enough air.
I think a dyno gain is for the filter only but to each his own. M would not leave 10hp on the table if it were a matter of tweaking the intake elbow diameter. Just makes zero sense