I built mine with 5 seats. I used flightline interior seats, with an extra center piece to bridge the gap, effectively making a bench seat in back. I used the existing seat belt attach points for lap belts, and a minor mod for the shoulder belts. I can convert back to a 4 seat in about 1 hour. I also built with 6 place intercom, and a 5th jack between the front seats for the middle passenger.
Aircraft was certified at Van's recommended gross, but as a 5 seat plane. This was after discussion with insurance, who agreed to insure the 5th seat for almost nothing.