I know this isn't what you are looking for as far as scale exhaust for an airplane. But over 20 years ago I made a few 'real' mufflers out of copper fittings for my gas buggies. Some of them I made with inner baffles and perforated tubes and the idea was to make a quiet muffler without losing performance and I succeeded completely. But the mufflers are indeed pretty heavy.

However, on a gas buggy they take a beating like an airplane would never see unless you re-kitted it. So for durability, I say yes copper is plenty durable.

The pictures are from when I took them out of a storage box recently.