Once the PowerFC is installed can I expect any better performance or does it HAVE to be tuned?
It is always recommended that you get your PowerFC tuned for your cars mods. This will take into factor things like a lazy fuel pump etc that will be missed with just the factory tune. A full tune will ensure your A/Fs are safe and the engine is not excessively knocking and you are getting maximum benefit from the new ECU. A standard map will have benefit over the stock computer as it wont have built in limits the factory computer has and it also controls things like Variable Valve Timing better (on r33 RB25det at least) and provides more torque in the rev range without a tune.
