The short answer is that I doubt if the PAP themselves know what is the actual return. You can calculate the actual returns, but its very time consuming.
First, you would have to track all the CPF money that was sold as SSGS to MAS. Than you would have to see how MAS apportioned the money among the 3 of them (MAS/Temasek/GIC). For example, if $100 billion was sold to MAS as SSGS for a certain year, did MAS park 20% with itself, than 40% with Temasek, and the remaining 40% with GIC? So you have to work this out first, and every year, they have a different apportionment %. I do not believe that it is fixed. I have to believe that the breakdown is at least 50% for GIC because they are the far larger fund. This complicated by the fact that CPF funds is not sold to MAS as SSGS once a year, but instead perhaps monthly or several times a year. So, you would have to trace it like a dye and follow it as it goes through the system.
Once you have this apportionment breakdown, than you have to see what each of the agency did with their money. Using the above example, lets say that MAS bought bonds at 5% with their $20 billion (20%). Temasek maybe parked their $40 billion into a short term instrument and than used it with other funds to buy a company. So Temasek's share is now blended with funds from other investment to buy the company. And lets remember they buy lots of companies a year. Once Temasek sells the company and puts the funds pack into a short term instrument, you can than tell what rate of return CPF's $40 billion with Temasek got. You do the same for the GIC's $40 billion, although they are not as active in the company buying business.
At the end of the day, you take a weighted average of all these returns to arrive at the overall return from the $100 billion CPF money for that year. E.G. MAS's $20 billion made $100 million in interest, CPF's $40 billion earned $200 million (pro-rated) after the company it was used to buy was sold for profit, and similarly GIC's $40 billion earned $300 million. And lets say all of this happened within one calendar year. Than simplistically speaking, the rate of return for 1 year on CPF's $100 billion would be $600 million or 0.6% return. this is just an example. In the years that Temasek and GIC lose money, it will be a negative return.
In my mind, this is the only way i would use to accurately calculate the returns, Unless someone on this forum is a financial investment guru, they can come up with a more efficient way of doing so. I welcome other ideas on it too. Good thread, and in my opinion, these are some of the questions that WP should have been asking in Parliament.