Firstly,because of the unique design of the brakes the standard iron rotors are lighter than CF brakes,so the unsprung weight goes to the standard brakes,but as usual the CF have longer running capabilites.
From the discussions we have had most people thing the modulation is similar because the modulation on the standard brakes is fantastic on the track,but similar on the road.
many of us have done track days and the standard are up to the job depending on youe style,we have just come back from castle coombe circuit and its the first trackday i have been on for many years (young family growing up) So being rusty was not even a consideration!!! After the first 2 sessions of 20 mins my brake pedal was getting long,although the stopping power was still there,but as i got into the groove in the afternoon the brakes got better because my usage went down!So if you track often the standard brakes imo are well within the job.But for alot longer sessions CF would be helpfull.
So when Mclaren tell you the brakes are up to it,they mean it!! And dont forget the airbrake plays a massive part.
Ultimately if the expense is irrelavant go for the CF they look great and keep your wheels clean,but for many it is a car that yo dont need to throw money on were you dont need to