Looking through the literature on happiness (those in the know say “subjective well-being,” or just SWB), it seems clear that a good number of those involved have egalitarian or welfare liberal politics. A lot of these folks profess to being utilitarians of some sort. And there seems to be a push for more redistribution, less inequality, etc. But I think I'm detecting something amiss, here.
Much of the upshot of the literature is that extra money doesn't do much for you; that people tend become accustomed to their level of material comfort; that people have happiness set-points to which they recur after positive or negative spikes in affect. The flip side of “a lot of money doesn't make you happy” is “not so much money doesn't make you unhappy.” So the problem with large economic inequalities isn't the happiness gap, because the happiness gap is small.
Now, it turns out that one's perception of one's place in the income distribution matters to happiness, such that people lower in the distribution are less happy in virtue of being lower in the distribution (or thinking they are). But, aside from total egalitarianism, which isn't likely to make anyone happy, there is nothing to be done about this. There is always going to be some distribution. There is always a bottom and a top quintile. The point being, I'm a bit puzzled at this point by the attachment to utilitarianism AND SWB research AND egalitarianism.
My hunch is that these folks aren't really utilitarians after all. They have a prior intuition about the injustice of inequality, and the justice of progressive redistribution. Then, they attempt to undermine resistance to higher tax rates on the wealthy by pointing to research that they interpret to say that this won't make the wealthy any less happy, and so, Why worry? The trouble is, it won't make the poor (in a country like the US where the poor are already rich) much happier either, and won't do anything to change relative position in the distribution. So what's the point? The point is more progressive redistribution, to which many folks are committed to prior to and independent of utilitarianism or their interest in happiness.
In a way, it turns out that dogmatic welfare liberals are just like dogmatic libertarians. I've run into a lot of libertarians who think that a perfect libertarian regime MUST be most conducive to happiness. Because if it wasn't, then that would be a strong argument against the perfect libertarian regime, against which there is no strong argument. Unsurprisingly, a lot of welfare liberals think this way too. Start with your political commitments, and then argue that everything good must revolve around your fixed point. This is fun at parties, but it tends not to make for good science.