I find this post from DominikTouring very thoughtful and I am grateful it was brought up!
We too have hit this.
It be great if Sonar could alter S3242 to only trigger on public methods or make it configurable.
In the future, Microsoft and/or Sonar could/may create a complimenting rule for CA1859 (or enhance that rule) to work on arguments of private methods to prefer concrete types. I think this would be great for IEnumerable/ICollection use, for example. The performance boost may collectively prove interesting.
Curious and hopeful someone from Sonar has some thoughts.