Have later YCombinator classes had a larger percentage of homeruns? If you have more applicants and have gotten better at selecting, this should be the case.
Is it about selecting winners, or weeding out losers? Are there companies that you know will fail? Who would be least likely to succeed as an entrepreneur?
Home runs are so rare that it's not a matter of percentage per batch. A batch will have 1 or 0, and probably 0. It will be a few years before I can tell if the rate is increasing.
Selecting winners and weeding out losers seem the same thing to me. There are companies we think will almost certainly fail, but we can never be sure. An ineffectual person would be least likely to succeed as a startup founder.