Before deciding to buy the RCF I have also driven both on the racetrack and on the city roads the C63 and the M4 and I kept an open mind along this whole process.
So my opinion on the RCF is:
Let alone that the sound of the RCF is simply amazing vs the C63 and the M4, The RCF is certainly not a car for drag racing and yes is a few cents slower from 0 to 100, but on the other hand, out of the 3 is the less nervous to drive, the more precise and (most important to me at least) the fastest on a single lap on a race course. Once you get familiar with the settings (quite easy to do), the RCF is a new world of fun to drive with plenty of 'controllable' oversteering if you like this type of driving...and I do!
So I don't agree one bit with the review given on the RCF on CarAdvice, perhaps the person who did review the car was not so impartial as I was, since at the end I wanted to buy a sport car which would give me great sensations and emotions while driving it and those 3 (M4/C63/RCF) were on my target list, with no prejudice.
Well what else to say about the RCF apart from the fact that I love to drive it and I love the feeling of power you have even at low speed believe it or not!
Buy and RCF with confidence and you won't regret it one single time in your life!