The output shaft in a 3/S is a critical driveline component and a common wear item. It is also a common failure point if the car is launched hard or often. Particularly the 18-spline stock output shaft is very weak and not recommend for launching, especially on a prepared surface like the track. When the output shaft breaks or strips you may overstress your costly VCU inside the trans and cause expensive damage. You may also find the car will not move once the output shaft is snapped or stripped. It is not recommended to drive the car if the output shaft is suspected of being bad because it can rapidly burn up your VCU.
For 5 speed cars it is highly recommended to use a 300M shaft. The 6 speed will also benefit from this, but it is not quite as prone to breaking. If you have to replace your output shaft, whether 18 or 25 spline it is recommended to get a 300M shaft or a good OEM unit instead of the aftermarket replacement shafts. The aftermarket shafts are somewhat weaker than stock.