It sounds like the steering column vibration under braking is probably due to either warped brake rotors or worn suspension bushings or components. The most likely cause is warped rotors. I will send you an article I wrote for Repairpal that we have not published yet.
I've had the same issue over and over again. I've replaced the rotors and the pads three times, once under warranty. I keep taking my G6 back to the dealership and they now tell me it's "normal" to feel that vibration due to tire ware???