What is the real ADC Sample rate for Xmega256A3BU or similar xmega controllers
You will find that it has "fast mode" options for the ADC peripheral, and that you will need to use lower resolution, like 8 bit, to get the maximum speed listed. The higher the resolution of the ADC the slower and more power used during conversion.

There is a whole application note on the use of ADC in the XMEGA series by Atmel by the way. AVR1300

From reading that briefly, it seems you need to have a XMEGA A series chip to get the maximum 2Mhz ADC clock rate, and even then to clock get 8 bit resolution samples at 2Msps the source impedance needs to be under 1k Ohms. See graph on page 14.