Copper is heavy...VERY heavy. Heavier by 1/3 than iron. I dont think you could brace it up enough to be nearly strong enough for racing. Especially once it gets soft from the heat. Even made out of good quality STEEL tubing of relatively thin wall thickness..a complete exhaust set up is somewhat difficult to stay bolted on and in one piece. And copper aint even CLOSE to cheap. I believe I would stick to regular materials. If there was any performance advantage maybe..but there is none I can think of. Myself...I would be trying to fab me up something way lighter.