I don't think the correct moral choice is about minimizing the total area under the pain-curve at all, it's about avoiding severe pain-spikes for any given individual even at the cost of having a larger area under the curve.

If you're going to say that, you'll need some threshhold, and pain over the threshhold makes the whole society count as worse than pain under the threshhold. This will mean that any number of people with pain X is better than one person with pain X + epsilon, where epsilon is very small but happens to push it over the threshhold.

Alternately, you could say that the disutility of pain gradually changes, but that has other problems. I suggest you read up on the repugnant conclusion ( http://plato.stanford.edu/entries/repugnant-conclusion/ )--depending on exactly what you mean, what you suggest is similar to the proposed solutions, which don't really work.