Orthogonal frequency-division multiplexed (OFDM) communication systems have become extremely popular in the last decade, and OFDM is often selected as the physical layer interface for emerging standards. A major drawback to OFDM is its inherently high peak-to-average power ratio (PAR), which can significantly reduce power efficiency and performance in multiple-input-multiple-output (MIMO) as well as single-input-single-output (SISO) systems. Active constellation extension (ACE) is a powerful technique for peak reduction in SISO systems. A MIMO extension was recently proposed, but lacked the computational efficiency for practical system implementation. This paper extends the efficient smart gradient project (SGP) technique for ACE to MIMO OFDM systems, and shows that it obtains sizeable PAR reduction with only two iterations.