The way it is balanced now, you will rarely consider building a second MkII or even MkIII Powerplant on one planet and this is intended, were the second MkII powerplant more efficient than the MkIII powerplant, everybody would first build two MkII PP which would make them a more efficient Mk III PP. If i remember correctly, this mechanism should force players to expand and to diversify.
Cheers,
K'Toff