The back seat is incredibly light. The back and bottom cushions are basically foam encasing a piece of sheet metal.
The seat belts, and the frame holding the seat to the floorpan, however, probably weigh 25lbs total. So...removing the rear seat components is maybe 30lbs.
Not having A/C really cleans up the engine compartment. My parents cars have all had it, my Explorer had it, and my Oldsmobile had it...not having it on the 'Stang makes working under the hood much easier. Also, as I recall, it cuts about three hours from a heater core repair.
