What is considered "high altitude"?
It seems I've asked a question that has no answer. From the explanation below provided by a Diamond Aircraft representative, it is clarified that the intent of the AFM's caution is not to always run the electric fuel pump at high altitude, but to use it as needed to clear the symptoms - which are more likely to occur as altitude increases.


  The IO 360 with the Precision Fuel Injection System does not have a
  vapor separation and return line to the tank(s). Vapor in the fuel
  system will travel through the fuel system including flow and pressure
  transducers, into the injection system. Vapor can form under higher
  ambient temperatures and low ambient pressures. Fuel gravity fed
  through the system (electric pump off) from the tanks is “lifted” by
  the mechanical pump. The suction of the mechanical pump can cause some
  cavitations (vapor formation), especially as the fuel flows through
  resistances such as elbow fittings. Vapor bubbles will cause momentary
  low pressure indications and may also cause momentary erratic or high
  flow readings as the flow transducer turbine spools up with the loss
  of fluid resistance. These momentary occurrences can be suppressed
  with the electric pump. The electric pump activation increases system
  pressure to compress or return to liquid, the formed fuel vapor
  bubbles.
  
  [...]
  
  The intention of the steps for electrical pump selection, above each
  of the Cautions in the AFM is not for continuous operation of the
  pump, as this could potentially mask a defective mechanical pump and
  eliminate the redundancy of the fuel delivery system. If the abnormal
  fuel pressure or flow indications do not stabilize with the electric
  pump on, emergency procedures and maintenance action may be required.