Just to share my experience:-
I did use my SA some yrs' back to buy a guarantted fund which
earned minimum 5% returns, ie, very safe and sure-win. It has since
matured and happily returned to my SA 
BUT, the funds are now stuck in SA; meaning my SA balance is now
way beyond the minimum required n will remain there for good.
Sorry to digress but some other "interesting" lessons I've learned
abt my CPF fund are:-
* Even though your company paid your CPF by the 14th of each month,
it will be credited to your a/c only about 5 days later. For my
case, the 19th. So on top of the 5 days' lost interest, you may
have to ensure some lost investment opportunities during that
period.
* Interest on your CPF a/cs is based on the LOWEST balance of the
month. Say if u happened to use all your OA balances on the last
day of a month, the interest for that month will be ZERO!
So plan your usage properly.