Good question. I think this verse argues against the "perserverence of the saints" prong of Calvinism. I think we must note here that while dogs return to their vomit, sheep are led back to the pasture.
The question is not really whether the sacrifice is sufficient for even these, but whether or not it is effective for them. If you reject the sacrifice, then the sacrifice will not do you any good. If someone offers you a million bucks it isn't going to do you any good unless you reach out and take it.
I'll take it your implicit answer is yes, it is still sufficient for them.