After I finished soldering all components and connected HiFiBerry USB to my Raspberry Pi I did some real-world tests.
The card is working, the sound is much better than the onboard sound if the Raspberry Pi. However I noticed, that the post-filtering of the DAC output of the PCM2903 is not very good. There are still a lot of high frequencies in the output. While nobody can hear these frequencies an active 2nd order lowpass filter after the output stage might be a potential improvement. There is still some noise on the output of the external voltage regulator. This needs further investigations.