The MED9 ECU's (MK5 2.0t etc) will switch to stock when the battery is disconnected yes, the newer cars fitted with the MED17 ECU (MK6, Rocco etc) will not switch to stock with a battery disconnect
the reason we did this with the MED9 ECU was it was something that a lot of our customers asked for at the time and thus it was set in place, we have not been asked to do this with the MED17 ECU and thus haven't
A good few miles I would've thought, although I can't really comment on VW ECU's - This is when a battery disconnect for 30 mins would help as it clears all the stored values and allows the ECU to start again.
Off to text my friendly VAG calibration engineer...
Doesn’t need to be 30mins, 30seconds will do it on these ECU's