Compared to a properly engineered single pipe? I don’t know. A second pipe obviously doubles the gas flow capacity, while doubling the weight of the exhaust system. Making a single pipe about 41% larger in diameter also doubles the cross sectional area of the pipe while increasing both the weight and internal wetted area by only 41%.
I get your point, with attention to all exhaust component engineering chances are the results are minimal. Going back to early comments the exhaust systems of 50/60s V8s were quite restrictive with single exhausts,
horsepower gains were obtained by adding duels,
On my car I did it strictly for looks and sound, I was rear ended, so used the opportunity to change the bumper cover to a GT cover and add a cat back with X pipe system.
Fun Fact. On the S197 GT’s the stock system really isn’t a liability until you get up to around 450+ HP. It has mandrel bends and is even oversized for the application. It’s one of the reasons axle-back exhausts are much more popular than a full cat-back for these cars. And even then it’s done for sound rather than any power gains. The S550’s see minor gains with cat back systems, provided they delete the suitcase resonator.