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Not sure 20's ion all 4 corners will work with the TC and AH. you need a stagger front front to rear of i believe .5 inches. That is one of the reason for the 2 different size wheels.
Also if you live in an area that has bad roads with pot holes and such, with the 20's you lose a lot of side wall on the tires which gives you a better chance of wheel damage.
Finally, if you have a base C6 the bigger wheels can look more like a wagon wheel then a custom wheel and that is base on the style of wheel with the small brakes.