Sorry for an indirect answer...
You probably won't get better than say prime - 1% on the bonds.
So that's around 8.25%
Now if you were paying off your main property (on which there's no rental income):
Not an exact calculation, but if you're paying a marginal tax rate of 40%, then you need an alternative investment that earns .0825/.6 = 13.75% interest before tax to be in the same position as dumping extra cash into a bond on your primary investment..
On a secondary investment (rental property):
You can offset the interest proportion of your bond repayments against your rental income. (and levies etc)
Thus it makes much less sense to dump a larger proportion of cash in here.
It's not too tough to find alternative investments where you will earn better than the interest you're blocking, and in addition, with a larger deposit, the interest part of your bond repayment becomes smaller..
Example: You have a rental property worth R800 000, repaying around R6000 a month, of which about R5800 a month is interest.
If you rent the place out for R5500 a month, you won't pay any tax on your rental income, because it's less than the R5800.
Say you choose to only have a R400 000 bond on it, repaying R3000 a month (R2900 interest), you would have to pay normal income tax at your marginal rate on the difference between the rental income (R5500) and the interest on the repayment (R2900) = R2600
Thus if you're at a marginal rate of 40% you would have to give SARS R1040 of it each month, which makes it less worth it to have the property.
Last thing, and I'm not a financial adviser or anything, but diversify your assets, consider buying equities, include some foreign exposure as well..
So my answer in short:
Take as close to 100% bonds as possible
Rather put cash into your primary property (where you stay and have no rental income)
OR use the cash to buy other classes of assets, like equities. Just get proper advice on which equities to buy..