In C6-9 Districts within the Special Lower Manhattan District, the regulations of Section 32-655 (Height of signs in all other Commercial Districts) may be modified to allow a maximum height of 50 feet above curb level, provided the City Planning Commission certifies that the design features of the existing building, as they appear on May 9, 2001, would unduly obstruct the visibility of the sign without such modification. An application for such certification shall be filed with detailed plans showing compliance with this Section.