Well, we can use the outside view here. If we look at people who are particularly successful, did they get that way by helping others? What's the proportion relative to poor people?
I don't think this backs up the idea of enlightened self-interest very well. Sure, you have to "play by the rules" to be successful, but going above and beyond doesn't seem to lead to additional success.
Another question we might ask is "where do peoples' instincts for giving come from?" If you believe Dawkins et al., it's the selfishness of genes, which does not have to causally pay of for the organism (instead, the payoff is acausal). This is not the sort of thing where giving according to our instincts will lead to us getting more money.
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