Over-declaring means you pay a lot more CPF (including MediSave) as well as a lot more income tax. Bear in mind that PAP will continue to make CPF and tax increasingly 'progressive'.
And, as others have pointed out, it's very dangerous living on credit, or even transacting on credit. If an investment goes wrong, triggering a domino effect of losses, it's the beginning of the end for you.
And one other risk of over-declaring : if you should one day decide you can't tahan the higher CPF and taxes, and decide to stop over-declaring, the CPF and IRAS may become suspicious and start investigating you, bringing in CPIB and CAD, in which case all kinds of other worms may spill out (eg. they find porn on your home PC which they confiscated as part of their investigations), and you could get charged and imprisoned for something completely unrelated to the original act of over-declaring.