Often when I release the gas-pedal, the car does seem to have a second-long delay before the slowing down kicks in. Do you also experience this problem? In my experience (talking to other S3 drivers) all the drive-by-wire engine cars manifest this problem. Is there any solution?

I bet it is the DV as well, Our A4 does the same .It can take several seconds before the engine revs start to decrease.
Technically I think that boost is still getting into the inlet manifold via the Diverter Valve ?