Universe as a counterfactual or hypothetical object seems intuitive, but claiming that it doesn't make a difference if you torture two or one identical beings in identical ways seems to be a bit problematic here. I think there are two ways to understand this, given two ways you can interpret the word "existence", as in, location within a given universe, and this metaphysical "does universe exist at all" -sense, and I also think that ethical worth of happiness or sadness can add up in both ways without this argument failing.

http://www.nickbostrom.com/papers/experience.pdf

This paper by Nick Bostrom seems a bit relevant, though I'm not implying it supports my claims.