Sorry if I'm a bit Johnny-come-lately.....but this is a discussion that's very near-and-dear. My friend and colleague Mark Segal directed me here as he thought I might find the discussion "interesting" based on some questions I was asking about a certain 1951 USAF target and it's real resolution.
Anyway, I've been rather immersed the past several days in finding good solid information on the resolution of various films, how this translates into scanning resolution, real detail ("system" resolution) and so on. I'm coming at this from a slightly different direction as I'm interested in how it relates to drum scanning....you see, I have a Screen 1045ai drum scanner in my office that I occasionally entertain myself with along with a gaggle of medium format gear.
What REALLY brought this to a head recently was a local photography colleague and one of the organizer's of our local Charlotte Photography Meetup Group that wanted me to try scanning a 4x5 sheet of something called ADOX CMS 20.....some sort of microfilm that's sold in sheets and 120 rolls. This film is claimed to have something like 800 lp/mm or 40,000 [cough] [gag] ppi resolution. Well, my scanner can only crank up to 8,000ppi, well under what the film is supposedly capable of.....but I assumed that the system rez (lens basically) would poop out long before we hit my scanner's top speed of 8K ppi. What I found was interesting.
One of my tests was scanning an area of small detail (fabric texture....attached) at 1K, 2K, 3K, 4K, 6K and 8K ppi. Similar to Mr. Van der Wolf's findings, I found detail improved quickly up to about 4K ppi with smaller improvements after that....but improvements nonetheless. What really surprised me was that there was real image detail improvement right up to the 8K limit of my scanner. Now, I've seen "improvements" before on my own films (Ilford Delta 100/400) but once past about 4K, it was really only improvements in grain structure, not actual image detail (side note....when I did tests of my own film, it was shot with my Pentax 645NII...I plan to do another test with my Pentax 67II to see if I can get some improved image detail at resolutions about 4K). In this case, it wasn't simply improvements in grain rendering (there WAS NONE with this film!). So either my colleague was using an excellent lens (likely) or, given the right film, lenses can perform better than I'd been led to believe (I hear numbers like 80-100 lp/mm bandied about for lenses).
Some fun facts about this drum scanner....
It has multiple discrete drum speeds that corrolate with resolution.....the higher the resolution, the slower than drum has to spin in order for the system to handle the data output. The "system" in this case is an aging Graphite PowerMac running the 90s version of OS 9.2.2! The drum speed is not stepless so there's always a transition point from one rez to another. For example, at 4000ppi drum speed is 900rpm....@ 4001ppi, it drops to 600rpm. What was interesting is that the quality slightly improves at the transition to the slower speed......at the limit of the faster speed (4000ppi) there's some grain "smearing"...but when you set the rex to 4001 and trigger the drum speed reduction, grain sharpens up. As a result, I typically only scan at 3001, 4001 and 6001 ppi resolutions.
This scanner has a 16,384 lineal pixel limit. So, for a 4x5" sheet film, your maximum scanning rez is 16,384/5" or 3,276ppi. So, if you really need a 4x5 scanned at, say, 6,000ppi, you have to cheat by scanning the sheet in sections and stitching it later in Photoshop...works like a charm.
Scanning time.....for a 4x5" sheet @ 3000ppi, it takes nearly 90 min. to complete the scan........and for a 6000ppi scan in sections, it took me nearly 9 hours of scan time! The one saving grace is the excellent batch scanning capability of the software. You can basically batch up a drum full of scans (or sections of a 4x5) and walk away for hours....or overnight in my case.
In an interest to staying on topic and perhaps adding to the discussion, I'm more of the mind of scanning once at the highest practical resolution (typically 4001ppi) and then downsample for printing. Since it takes something like 35-45' to scan a 6x7 format, I'd rather do it just once.

My own personal workflow is, after development, I sleeve all my negs and scan them on my Epson V750 (yes, I have one of those too)....that becomes my contact sheet. After looking through the images, I then pick out the best (typically only 20-30% of each roll) and go through the process of wet-mounting and drum scanning.
Regards,
Terry