sub definately worth it but a pita to get added in to the standard system as theres no dedicated subout. DSP unit (if you have it on you model) might have sub out but i could never find a pinout for it to confirm.
depends if you've a model with the DSP or not will depend if it's worth upgrade the rest of it too and then the sky's the limit with how far you take it
DSP unit is behind the plastic directly to the side of clutch pedal in the center of the car.
premium audio was an option on non-GT, and standard on the GT. GT's are easily determined by the red strip across the front (although some add/remove it) and dual-zone heater which afaik was never on the non-gt. front park sensors also a GT only thing i think.
cant say i've noticed distortion on my setup although it is slightly tweaked from standard.
I have mine in my gt set to +3 bass & -1 treb