The interest exemption is 23.800-00 (<65) not 31.000-00
So you can have an interest bearing investment(s) of 340.000-00 @7% = 23.800-00 and pay zero tax on the 340.000-00
Taking the NBKP pref. @ currently ± 7% you would pay tax of 3.570-00 (15%).
If you double the invested amount, and assuming a 30% marginal tax rate:-
Interest bearing investment(s) is 680.000-00 @7% = 47.600-00 less 23.800-00 (exempt) = 23800-00 taxable @ marginal rate 30% = 7.140-00 tax
NBKP pref. - 680.000-00 @ ± 7% = 47.600-00 less 15% = 40.460-00. Tax also 7140-00
So the higher your income and marginal tax rate, the larger the benefit in favour of pref. shares.
Pref. shares are best suited for high net worth folks and are for the longer term. Their attractiveness increases in a rising interest rate cycle, when they should outperform bonds. Capital gains (and losses) can be made.
See also
Here