OK here's the scoop. The upgraded backing plates are needed if you go to anything bigger than the stock size bar for your car. The reason being is that the backet material that the loop hooks into is lightweight material and can easily bend under the higher loads placed on it by increasing the size of the rear bar. In your case you may have damaged your brackets in the accident if the rear tires were pushed sideways on the concrete and/or contacted the curb as well. If you don't plan to autocross every weekend or install a larger rear sway bar, the stock pieces should be fine for the repair. If you have the ability to look at it for yourself before you take it to a mechanic you should be able to tell whether or not the brakect is bent and/or the bushing retainer has pulled out of the bracket.
HTH,
Rick
