Firstly, as I've been saying, to view what you get from social security as a rate of return on an investment is to fundamentally misunderstand the purpose of the program.
Secondly, to believe you (or Greenspan) can possibly know what that MEANINGLESS number is if you insist upon calculating it requires a forecast of future payments, benefits, inflation, etc. It is impossible to be certain what the "return" will be or whether it will or won't exceed 2.7%.
We calculate future payments, benefits, inflation etc. everyday. Thats the stock and bond market.The problem is that we calculate them based on a companys ability to make a product, earn a profit and pay dividends and interest. We have some insurance based on the company's assets. Social Security is based on a politicians word that I'll be paid in the future. No thanks.
Like I need another reason to want to keep my money out of that system.