Just wondering if anybody has ever done any modifying to their Grand Prix'* brakes? Of course mine is an 06' GT
Like adding dual piston calipers, and 12.7" Powerstop Rotors and pads?

Most mods mention front, cant find any for the rear..

https://zzperformance.com/products/1...ston-brake-kit
with longer brake lines..
https://zzperformance.com/products/g...12623346303087

Those would do good on the front, but I already have new rotors and z23'* waiting to go on my front, so at most I could upgrade the front calipers, but wait on the 12.7" rotors, then again I am not sure if I can find those dual piston calipers by themselves, and not in a set...

Unless you're really really hard on brakes, I don't think it'* necessary. They're pretty good stock in my opinion.

The rears definitely don't need to be upgraded, they barely do anything.

So my power stop Z23 pads and drilled/slotted rotors should be plenty of stopping power...
I read somewhere about a person saying the brakes on the Grand Prix'* are barely adequate, which is why some upgrade to 12.7: rotors...
I think they will do, I'll probably be doing my rear, as long as I can get the parking brake right...

Depending on how you use them, yes.

Opinions vary. Maybe they are right. Maybe they are just justifying why they paid a grand for a wicked cool brake kit for their car and it'* their story and they're sticking to it.