i have to disagree with you on this one formula. i jumped from stock '75 bbp discs to the viper conversion from AR and even with the small 11.75" rotors the difference is dramatic.
The factory a-body set up allows "Part is in stock" (calipers, hoses, standard pads, rotors) maintenance at any parts store. The after-market kits normally come with good "street performance pads", something that is often overlooked when doing a factory swap.
Can I get a show of hands from people that did the factory swap and went with standard consumer quality pads...
The thing many are feeling with an after-market kits is the pad quality.
(Until you start getting into the big kits that require larger wheels and so on... but thats another story)