Setting non-CPU lens data, even if incorrect, enables Matrix metering. Having correct lens data my give slightly better exposure calculation from the Matrix evaluation. The improvement using correct data would probably be most noticeable in very tricky lighting situations. For Center Weighted of Spot metering, the lens data has no effect.
If your experience is with matrix metering, you have demonstrated that even with extreme mismatches between the lens and data setting, the metering frequently is very good. This works, because the meter reads the light with the lens wide open and then corrects for the number of stops difference for the actual exposure setting. The camera really doesn't do much with the actual aperture, only the difference is important.
I frequently use my MF 400/3.5 Nikkor with both 1.4X and 2X TC's. I just use the 400/3.5 setting with the TCs. In my experience the exposure is right on without telling the camera about the TCs.
The most important advantage of setting the corrected focal length and aperture is to get that information correct in the EXIF data.
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