The rise and fall of rental goes together with property prices (with some lag). Thus, if you feel that rental is high, so will the property prices.
Firstly, you're idea of using CPF to pay mortgage rather than to use cash to pay rental is a viable plan. You will be acquiring an asset, but at the same time, you are also acquiring a liability.Buying a property in Singapore as a sound investment may be debatable. It depends on your situation. If you plan to put a majority of your asset into property, it will be risky. Property investment is not liquid - meaning, if you need cash all of a sudden, you may be forced to sell the property at a loss. The up and down cycle of property is long (up to a decade). Overall, I believe property appreciation will at least keep pace with the inflation rate. Investing in a well researched global equity has proven to give much better yield than property.