I've never seen any proof, and at 30 years old it would be hard to come by. Generally speaking I think the 79-80 cars are lighter, but also remember that the pickup points changed in 84-85 which I think is a greater advantage.

There will obviously be a weight difference in sunroof and non-sunroof cars.

The biggest and most often overlooked bit of weight savings in first gen RX-7's though is wiring. The whole wiring harness weighs OVER 50 pounds, and you actually need very little of it, especially if running data or aftermarket gauges.