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Yes, adding a larger rotor adds unsprung weight, and the difference between a 321mm stock rotor and a 345mm Caprice rotor is about 3-1/2lbs.
On a race car that could be significant, it might also even be an issue if it was that much of a variance from one side of car to the other. But only if you had an incredibly well calibrated butt chassis dyno and drove the hell out your car, Most people have such numb hands and ass that they can't even tell an underinflated tire, never mind 3.5 pounds of additional unsprung weight.
And of course since virtually no one shops for tires by weight, the fact that your unsprung corner weights could vary by well over 3lbs just by a tire change negates this issue as a reason to justify going with a smaller rotor.
That benefit of the extra weight of the 345mm rotor gives a larger "lever" for the brakes to act on, larger swept area, a larger pad, and additional mass to dissipate heat.
Adding caliper piston area will increase pedal travel and reduce pedal feel. In other words you will get the same brake pad force with less pedal force, but with longer pedal travel. I don't consider increased piston area an upgrade. You may if you like long, soft pedals.
Of note, switching to a 4 piston caliper does not necessarily increase caliper piston area... The ATS 42mm pistons arranged in opposed pairs actually equates to slight LESS piston area than the stock 60mm single floating piston in the Impala. (4.31"² vs 4.37"², less than 2% variance). 4 piston calipers don't apply more force just because they have more pistons. Compared to a sliding caliper of the same area, any difference in performance comes from increased rigidity of the caliper. We've got serious cast iron caliper, they don't suffer the same flex as a floating alum caliper does. I have no reason to believe there would be a performance advantage resulting from a fixed caliper.
345mm rotors fit in stock 18's. This car should have had the 345s stock, not the medium car 321s.
Dropping $300 on an ATS caliper setup is a waste of money and time for any potential fabrication, from a performance upgrade point in my opinion. A simple pad change with a well researched performance pad upgrade will change the way your brakes perform and feel.
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