Thank you for the reply! That was my first thought as well, but I can't really find a clear cut answer as to whether or not the sDrive23i does or does not have the 3 stage manifold.
Looking at the realoem parts list for the intake system for both the 23i and the 2.5si, I don't really see a difference.
23i:
https://www.realoem.com/bmw/enUS/sho...diagId=11_3756
2.5si:
https://www.realoem.com/bmw/enUS/sho...diagId=11_3756
To me, that implies the 23i already has the upgraded intake. I also imagine this mod would be more common if it were indeed a possible upgrade, since it seems one of the best increases in power you can get from the N52.
If anyone has an idea where these additional 15-ish hp come from, I'd be glad to hear it