Get the S without a doubt.

 

No Same exhaust as standard in U.K. On non S, also someone told me exhaust has two modes on C63 and three modes on the C63s this may explain the louder noise in race mode I thought I heard as detailed above about flap control in race mode.
Tried today sport + then race mode defo sounds louder and more snap crackle n pop on down shifts

 

Sure! Mercedes-Benz AMG C63 with engine M 177 Type 205 AMG Introduction into service, Daimler AG 9/2014

It is not engine performance itself, but - as you said chassi and engine response (especially steering response as well!). Page 58 of the document above.

Page 60 of the document above:
"AMG Hochleistungsbremsanlage C63...Bremsscheiben an Vorder- und Hinterachse...360mm"

My translation: "AMG high performance brake system C63...brake rotors front and rear...360mm" (including pictures!)

"AMG Hochleistungs Verbundbremsanlage S-Model...Verbundbremsscheibe...390x36mm an der Vorderachse...Dual-Cast Bremsscheiben bestehen aus 2 Materialien - Gusseisen und Aluminium - und bieten verschiedene Vorteile, wie eine Gewichtsverringerung um 50%, einen höheren Komfort beim Lenken und eine bessere Bremsperformance"

My translation: "AMG High Performance Composite-Brake-System S-Model...composite-rotors...390x36mm in the front...Dual-Cast rotors that consist of 2 materials - cast iron and aluminium - which offer different benefits, such as a 50% weight reduction, higher comfort at turn in and a better brake performance"

As I already wrote: The exhaust is the same, but the handling of the FLAP(s) differs between the modes!

Page 47 of the document above:
"Abgasklappe, die je nach Leistungswunsch des Fahrers und abhängig von der Motordrehzahl elektrisch vom Motorsteuergerät...kennfeldgesteuert betätigt werden."

My translation: "Exhaust flap, that is electrical driven and computer-map-controlled, depending on mode, rpm and throttle position"

Without Performance exhaust, you still have the flap in the H-pipe, which is depending on drive modes, offering the smoothest ride in "Comfort" and the most agressiv in "Race". With the Performance exhaust, one adds two additional flaps in the rear muffler. Those can manually be opened via the push button - but the H-pipe flap remains computer controlled and dependend on the selected mode.

Just give it a try: Let someone drive your C63S. Ask him to do a few drive-by starting at 1.500 rpm in 3rd gear with full throttle (gearbox "M" AND Performance Exhaust flaps OPEN). Now do that in "COMFORT", "SPORT", SPORT+" and "RACE". Even though the exhaust flaps pushbutton light is on all the time (as you had pressed it), the sound differs a lot. For me, "Sport+" sounds the best, as "RACE" is to deep...to raspy for me.

 

Thanks for all your research and info.

Curious though... Am I to understand that in order to achieve maximum sound from the performance exhaust, the gearbox needs to be in "M" mode, for manual, and not "D" which will shift for you?

Also, I never have to actually press the button to open the flaps... I never drive in Comfort mode... So it's automatically always pressed.

 

That's really interesting. So a good exhaust mod if you want a more aggressive exhaust note is to permanently close off the H pipe?

 

the grey metallic paint is over a thousand less on the S model.....then toss in the steering wheel, less expensive carbon fiber trim, zero charge for leather seats.....the difference is materially less (if the paint color is what you're after).