The last Datasheet of 128A1 gives the internal 32MHz an accuracy of +/- 1.5%, the internal 32.768kHz +/- 0.5% - all at 85°C (185°F)!!
Over the Temp-Range you have another 2% for the internal 32.768kHz (see Fig. 35-21 of Datasheet). Assuming a precise DFLL source you can have an accuracy of max. 0.2% (stepsize of DFLL).
Short story: If you want to have a high precision timing with XMEGA go for a external crystal or crystal-oscillator and disable all internal oscillators.
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