Also keep in mind the income draw form the annuity is taxed according to the income tax tables. Thus in your later years you might be paying more tax then expected, which defeated the whole point of using the RA for tax reasons.
The second thing I would like to add is that the future value of the tax benefit might be worth less then you think. Assume you contribute a R1,000,000 over the limit this year, assuming a inflation rate of 6% over a 20 year period, that R1,000,000 will only worth R311,000 in present value terms in 20 years time.
These two factors together with the fact you lose flexibility in terms of accessing the investment and the fact that you already seem to be contributing to RA's make a flexibility investment option a more logical route to take.
Hope this helps.