Does increasing burn speed for CD-R above 16x sometimes improve burn quality? Yes, but most of the time burn quality is best around 16x or 24x with current burners.
Unfortunately, there are no currently produced non-professional drives that will perform Disc Quality scan with enough information about burned CD-R media - especially the lack of jitter information in such scans is problematic IMO, since there's a huge difference in jitter between good and bad burns/burners.
16x is very often (but not always) the best burn speed for CD-R media, since it's the fastest burn speed that is constant across the disc (CLV) and this usually results in the lowest jitter.
I have found that on my Pioneer DVD burners, 24x is the sweet spot, with possibility of some serious jitter problems when burning some CD-R media faster than 24x.
There have been reports of some DVD burners being very poor at burning CD-R media below the maximum burn speed; I'm guessing this is the result of almost no testing being done for CD-R media being burned at less than maximum speed in those DVD burners.
The best thing you can do is really to test your own media and burners as you have done.
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