In this case, if you said "John Smith has been known to be a great public speaker" I would think that you meant that he had occasionally spoken very well in public but that he had also given some terrible speeches.
- Do you eat spinach?
- I have been known to. (I've done it but I don't do it on a regular basis.)
- Is your boss generous with pay rises?
- He has been known to be, although he has never given me a particularly good rise.
- Is your dog very aggressive?
- He has been known to be, but normally he is very sweet.
If someone has historically always been a great public speaker, there is no reason to believe that that is any different today, so I would say "John Smith is known to be a great public speaker".
Student or Learner