Edit: In other words, I think Torture v. Specks is just a restatement of the Repugnant Conclusion.

The Repugnant Conclusion can be rejected by average-utilitarianism, whereas in Torture vs. Dustspecks average-utilitarianism still tells you to torture, because the disutility of 50 years of torture divided among 3^^^3 people is less than the disutility of 3^^^3 dustspecks divided among 3^^^3 people. That's an important structural difference to the thought experiment.