When I studied with Arthur Weisberg many years ago, he was at that time playing on a bassoon with a whisper-link from the high-A-key. I saw on the instrument evidence of a removed whipser-link for the high-C-key and asked him about it. He said that he had had both installed and found that the link from the high-A-key was a good idea, but that the one from the high-C-key did not work. I pressed him and he only elaborated that it helped some notes but interfered with others.
Since then, I have experimented with a temporary link from the high-C-key. Without question, the link improves the pitch, tonecolor, and response of high B especially in soft dynamics. However, it greatly interferes with high C and high C# where one uses both the C and D octave keys. As well, the three "flick" notes using the high-C-key are not as good, though not as terrible as the high C.
I have an extra whisper-key for the R-thumb on my bassoon and pretty routinely use it when playing high B. (R-thumb presses both the whisper-key and the Bb-key.) Of course I have to forego the R-thumb whisper-key in certain situations like going quickly between high C and B.
I have been working on a mechanical solution so that the whisper-key is automatically closed when playing high B, but not when playing high C, C#, nor the flick notes below. Once Frank Marcus installs it for me (and I determine that it works properly!) I will post a photograph and detailed drawings.
Prin. Bsn., Toronto Symphony Orchestra