I have read similar studies and find that amazing when considered in the light of the fact that successful people tend to be more optimistic. So, seeing a "clearer view of reality" doesn't seem to confer any advantages. I lean toward the view that intelligent people are more depressed because of the fact that they see reality more clearly.

The problem you are describing here isn't a self-employment problem, it is a project management problem. If you had created a project plan and built realistic time expectations with your customer, then he could not have been irate. He would have signed off on the project plan, the schedule, and would have been kept abreast of the project's progress via milestones and a strong communication plan.

This is going to be a problem whether you are self-employed or work for $MegaCorp.