This question can best be answered by looking at the evidence from 40 years of Delphi Research

Those enquiries that have been used to forecast out comes of future events, whether they are crystal ball gazing into the future outcome of technology, defence, strategic direction or sales forecasts have seen that Delphi techniques are ften able to be more accurate than either committee based forecasts, subjective forecasts or quantitative forecasts.

Delphi 3-4% variation

Quantitative forecasting 10-15% variation

Unstructured forecasts 20% plus variation

Source: The Delphi Method Techniques and Applications http://is.njit.edu/pubs/delphibook/ and The Delphi Process  Basu and Schroeder (1977), http://en.wikipedia.org/wiki/Delphi_method