Be aware that the salary sacrificed amount is 'grossed up' (using the highest tax rate) and added back to accessible income for all the 'extras' in the tax and benefit system. The grossed up max is 30k.
So your 'income' for purposes of HECS repayments, family handouts, levies, surcharges and child maintenance etc will be based on the 'adjusted income' figure.
Also, if your employer is paying the interest on your IP loan (through your salary sacrifice) then you will not be able to claim it as deduction on your tax return. You are much better off using the SS to pay otherwise non deductible expenses like PPOR repayments, phone, Internet and utility bills, credit card payments, personal loan repayments, etc.
Totally agree, there is no saving from an adjusted income point of view for benefits or HECS payments.
Another point to remember is that the 9% super is generally paid on ordinary times earnings. (Dependent on any workplace agreeemnts) If your wages are lower due to salary sacrifice, your 9% superaguarantee from the employer could also be lower.
The benefit is that sacrificing reduces income tax payable, as expenses are paid out of pre-tax dollars.
And yes, much better to salary sacrifice non-deductible items, as there is no benefit to sacrificing deductible items.