We can discuss what the QRP power level is for other modes.  BUT, there is really only one mode for normal communication with the QCX: CW.  QRP is well defined for that mode - 5watts.  This could change when (if?) the QSX is released.  Until then CW is the mode.

In my opinion, we do not really need to get any deeper than that.
73
Evan
AC9TU