The latter is clearly documented.
The former is not clear in terms of the magnitude of the difference. No one has compared the real world difference between holding the battery pack at 90 and 50% over a 5-10 year time scale. I strongly suspect the difference is smaller than most people think. Elon has said this, but Tesla has provided no data.
also, if brick balance gets bad enough, the entire pack becomes unusable and must be replaced.