Increase accuracy of ADC without device AGND
would I see accuracy benefits equivalent to a device that had
  dedicated analogue supply pins?


Well, the data sheet isn't that great. For instance it doesn't obviously tell you what would be the accuracies if you used the software selectable internal voltage reference so, ignoring that and basing my answer stricly on the question asked, the table on page 425 provides some insight: -



The basic accuracies are shown within the blue box and could be up to +/-6 LSBs. With a voltage reference of 3 volts (Vdd = 3) one LSb is 2.93 mV and so a reading could be accurate to +/- 17.6 mV.

If your input signal is such that it needs to use most of 3 volt span then you are stuck with an accuracy of +/- 17.6 mV. However, if your input signal might only range between 0 volts and 1.8 volts then one LSb is now 1.76 mV and you have an accuracy of 10.6 mV.

Generally it's better to use an external reference but, if that voltage reference is 3 volts then expect no improvement other than that due to it being stabler that the voltage regulator that produces Vdd.

However, if you are measuring a signal that is ratioed to Vdd then use Vdd as the reference because then you get the benefit of a ratiometric conversion (if needed).