My Stand Up Paddleboard came with a large Plastic Center fin, is that good enough ?

In short, not really, a decent large fiberglass center fin will is much stiffer and stronger. It will provide far greater stability and control for you when paddling and riding your SUP.  If your SUP has side bite fin positions, adding in these fins in place as well will also give you a noticeable increase in stability grip when you paddle or when you are surfing your Stand up Paddle board (SUP). The OLI SUP boards produced and sold by Numfish are set up with a 10” US Base center fin box so it’s a standard fitting that any US Base large center fin can be fitted into quickly and easily.  The Side bite fin setup are FCS generic fin plugs so any FCS tab base style fins can be fitted. Numfish have excellent value fins available and they are easily shipped anywhere in Australia. Jump online to Numfish and get yourself some decent fins for your SUP and you will notice the difference.