That sounds like a typical scenario of clutch slip! Normally tested by a high gear pull to see if the clutch can handle the torque the engine puts on it.
It's recommended for the whole lot to be done, clutch kit, flywheel, slave cylinder, braided line and new fluid obviously. The zeds have dual mass flywheels as standard which can fail, so a lot of people replace with an upgraded, lighter single mass flywheel which is cheaper and "revs up" quicker.
Not a cheap one by any means and I'm definitely not looking forward to having to do this to my Z