In South Africa's criminal code, what Americans tend to call "self-defense" is called "private defense" and your question really only implicates when different forms of private defense are available under South African law, and in particular, private defense of property. As Wikipedia explains:
Private defence of property
This defence is available when a person uses force to defend an
interest in property: for example,
to prevent a would-be thief or robber from taking his own property, or that of another;
to prevent someone from damaging or destroying his own or another’s property; or
to prevent an intruder from entering his own or another’s property. This would include the use of such preventive devices as spiked fences
and electrified fencing.
The requirements for private defence of property are similar in many
respects to those for private defence of persons, but there are
certain differences. The following are conditions relating to the
attack. There must be evidence that the property was:
The defence of property must be:
directed against the attacker;
necessary to avert the danger; and
a reasonable response to the attack.
In Ex parte Die Minister van Justisie: in re S v Van Wyk, the
Appellate Division held that the onus is on the State to rebut private
defence of property, just as it carries the onus to rebut private
defence of person.
The property should not be of negligible value. In S v Mogohlwane,
Mogohlwane had been robbed by the deceased, who had been armed with a
tomahawk, of a bag containing his clothing, shoes and food. Mogohlwane
then went to his home, nearby, fetched a knife and returned to recover
his property. When Mogohlwane tried to take back his bag, the deceased
resisted and again threatened him with the tomahawk. Mogohlwane then
stabbed him with the knife, causing his death. Mogohlwane was charged
with murder. The court held that, in determining whether or not the
property is of trivial value, it could be taken into account that the
accused (as was the case in casu) might not be richly endowed with
earthly possessions. What may be of little value to a wealthy person
may be of great value to a poor person. Given Mogohlwane’s financial
circumstances, the stolen items were of value to him. Mogohlwane was
justified in his conduct, because his attempt to recover his property
was close enough in time to the robbery to be part of the same chain
of events. The State had not proved that there was a less dangerous
and more effective means or method reasonably available to the accused
to defend himself against the act of robbery, so it was decided that
Mogohlwane had acted in private defence and therefore lawfully.
There is no real question in this case that justification for some private defense of property was present, and in both the scenarios presented, it is directed at the attacker.
So, the question under South African law would be whether the force was "necessary to avert the danger" and whether it was "a reasonable response to the attack."
Generally speaking "deadly force" is not going to be justified to protect property in a smash and grab, although in the first scenario when someone is in the car, there may be an implied threat to a person that would justify a greater level of force than would be the case otherwise.
There is, however, no easy answer. It is a judgment call that must be evaluated for necessity and reasonableness on a case by case basis, and probably has to be evaluated in light of other alternative options. If your 300 pound muscled bouncer friend could just grab him instead, it was probably not necessary to smash his legs or use a taser. In other circumstances that might be the best available option.
Similarly, under Mogohlwane it would be appropriate to consider if the victim had insurance and how much of an economic impact this would have on the victim and what the property being stolen was. Protecting one's life savings (or better yet, a medical device or medicines necessary to stay alive) might justify action that would not be appropriate when protecting a cheap pair of sunglasses in a car about to be junked anyway.