The main reason why Hindus are so far out there is that as a rule only top caste Indians have been able to emigrate.
Not accurate. I work in Silicon Valley in the high tech industry. Every boss I've had in the past twelve years has been Indian and at least nominally Hindu. Those I got to know well enough that I asked or was told their caste were anything from Brahmins to Sudras. Caste might have something to do with how much money a family has in India, but it does not have much direct influence on who goes to the top universities.
Family wealth, however, is a major part of who gets an education, and is therefore able to go to university. Among that group, performance in school determines who goes to the IITs (India's equivalents to MIT or CalTech) or an equivalent, and who goes to a second-ranked school. Of the people who come to America, performance at college is the major deciding factor in either gaining entry to a top-ranked US graduate school or obtaining an H1-A visa to work. (H1-A visas are for highly-trained people, usually with at least the equivalent of a U.S. Master's degree in their field.) In other words, wealth and academic performance are the factors that matter, not caste.
I strongly suspect that, if you control for a) education level and b) field, the amount of money made by Hindus and every other group that they examined would be explained. Entrepreneurs and engineers tend to make a lot of money; most Indians who come to America fit into one of those categories. If I look at the people I go to church with, the majority are as well educated as the Indian bosses and coworkers that I've had, but often in fields that don't pay as well. There are quite a few college professors (including one of my priests a few years ago), a number of teachers, several people who work for the government, and a number of people who work for a church or other small non-profit organization. This even applies to the two doctors I know at church: both work for a large managed medical firm rather than being in practice for themselves. Those jobs often require degrees as advanced as jobs in the high tech industry require, but they don't pay anything like the same salaries.
That doesn't mean that they aren't as worthy of respect as the jobs that do, of course. And *there* you'll find a key factor. (I agree with several others who have noted this already.) What the parents and community value often has a great deal to do with how much education a child gets, how hard they work at school, what types of college majors they choose, and what fields they end up working in. If we Orthodox value fields that don't get the highest salaries, but put a great deal back into the community and improve lives, I think we should hold our heads high. Making a fortune honestly is nothing to be ashamed of, but failing to do so because you chose to teach the next generation how to read and write is also nothing to be ashamed of. And choosing to be a priest and working for a minimal salary at a small mission church (as many of our priests do) is one of the most honorable professions I can imagine. But anybody who does it to get rich is obviously not smart enough to be a priest! ;-)