You are not experiencing buffer underruns, but the prevention of same. You would not be aware of this if Nero didn’t make this report. Two instances of buffer underrun prevention in a full burn is not a “continuous use of buffer underrun prevention”. Making your own definition does not change facts or the established and commonly accepted definition of a term.
If in your opinion, your computer is fully capable of 24x burning without buffer underruns, so be it. The fact that buffer underrun protection is being used at 24x speaks for itself with no regard for opinions.
The fact that buffer underrun prevention doesn’t occur at 16x is further proof of the relationship between burning speed and buffer underruns.
Buffer underruns are rarely related to hard drives, and never on a newer one that has been defragmented.
At a 24x burning speed, the Plextor buffer has 1.1 seconds of data storage. Rather than build in a larger buffer (old technology), Plextor uses BurnProof with the full knowledge that it will be required and used. You are chasing after a nonexistent problem because of an irrelevant Nero message that no other burning software bothers with.
Nero probably included this message to indicate that buffer underrun prevention is working as expected. If you choose to see it as a problem, so be it.