I agree that adding excess weight at the transom is usually not a good idea - particularly a boat that operates at planing speeds. If it were me I would stick with the two layers of 3/4". If I was concerned about strength I would add a "beam" at the top of the double layers. This could be something like a solid 2" x 4" set on edge running the width of the transom. It would make the transom more rigid but not add a lot of weight. The drawing below might better explain what I mean. It is a side view with the red square being the beam.
Or even a 6" wide piece of 3/4" plywood attached to the top of the double layers like the drawing below would help add stiffness.