Size is O(N) whereas an eq check is O(1). So it does make a difference for hashsets with lots of collisions, which happen occasionally if you don't have a good hashcode. I had some performance tests with keys that are specially designed for lots of collisions that would benefit from being able to to an eq check.

But I agree that it is probably unwise to rely on the behavior of ListSet without having a test to ensure the behavior, so I will remove the comments for this PR.