His car has no roof. That chassis will flop around before teh suspension is fully loaded. That's why I suggested the subframes. The little plates the factory used under the car help a wee bit but not much.
I bet if he loads that car into a corner it will plow horribly no matter how much throttle is applied.
If he slings it into a corner and mashes the throttle it will probably come around on him due to the twisting chassis unloading the inside rear tire and allowing wheelspin.

The rear bar should make a nice improvement regardless of the subframe connectors. I doubt he's going to be road racing it any time soon.