The factory roll bar is sufficient for most AX and track day events (accepted by PCA, POC, etc) because it is hard-mounted to the chassis in four places - which is exactly how an aftermarket roll bar would be installed.
If required or desired, you can add a Brey Krause roll bar extension to the factory roll bar to get a few more inches of head clearance. I have one of these installed in my car.
Brey-Krause Roll Bar Extensions for Porsche
If you want to go wheel-to-wheel racing, then most organizations will require you to go install a full roll cage.
Check with the orgranizers of the events that you'd like to attend to find out exactly what they require (you may already meet their requirements with the stock roll bar).