From what I have learned about the electrical system on these cars, the frame should never be used for an electrical ground.
The main battery ground goes to the engine as everyone knows, and also there is a ground branch attached to the body. It is vitally important that any current path being supplied voltage MUST also have the same ability to return the to the battery.
Critical engine sensors have this direct path back to the battery, and other items like lights, take a path through the body to the battery.
The low carbon steel frame made up of many pieces bolted and welded together is a very poor conductor, and it gets worse with age and corrosion. Compared to a length of stranded copper wire, it is NOT a proper conductor in any electrical circuit.
Some owners may say that "the frame grounds work ok on my car", but when performance is increased, supplying AND returning the circuit to the battery, can and does, become critical, and can give un-wanted issues.