Regarding the CSC and CMC....the OEM ones are crap...Of course if you don't drive the car hard it's bound to last a lot longer, however its not a guarantee....A friend of mine who has a 2013 model had his fail a few months ago...Car didn't even reach a year old....This friend of mine does not drive his Z hard at all and babies the hell out of it....More of a hard parker.
In regards to replacements, Zpeformance also sales a HD CSC Replacement....The techs at Nissan were really impressed with its overall material and craftsmanship when they installed it in my 1st Z34.
ZSpeed Performance Replacment HD Clutch CSC Heavy Duty Slave Cylinder Kit