You asked me to post Behringer's response so here it is. Unfortunately they weren't much help. Here is what they said:

Hi,
and thank you for writing to BEHRINGER technical support! You're correct, the ECM8000 is designed to be used with a phantom power source as low as 15V. We don't have any information regarding how the ECM8000 will function with less then 15V. It doesn't sound all that far off in theory, but basically it's not within the specified range of the mic that we know WILL work well.