Armed - It's not the back of the seats where the main weight is at, it's all the metal crap under the seats....Take a look directly under the seats and you will see little gears, motor, and alot more heavy metal stuff...That is what I am talking about...Just gut it all out while maintaining the structual integrity of the seat and frame..... Keep in mind - The gears and motor move the seats forward and back electronically, so if you remove the guts, that means you will have no more seat movement...OUCH!!! That is why you would need to weld in or screw in hand adjustable slide rails... May even get more leg room with the change... It's alot of custom work, but with a little patience, measurements, some old seat rails I think it is doable.
The problem with after market seats like the Recaro's and Supra's you will "NEVER" ever find a perfect interior color match... You will instantly ruin the stock look of luxury... The stock rear seat will never match in color with the new fronts.....
Do both seats and I would venture to guess you may drop 30-50 pounds... The reason I say that is that I had my seats re-done, really there is just foam and leather throughout, the skeleten frame looks very light in weight...So that leads me to believe the real meat and weight of the seats are underneathe... Maybe more then 50%...