I agree that the best option at this time would likely be only changing the mufflers. Though, hopefully in regards to the exhaust manifold, nobody is screwing with the exhaust that far upstream.
Keep the exhaust diameter, or close to it (after the y split), with a good, high flowing muffler. This shouldn't result in changing enough flow dynamics to shift your torque curve much. Instead, you'll mostly get that exhaust sound you're looking for. If you know a good muffler shop/welder, I'd say take a stab at it. Nearly any aftermarket option, once available, will be a cat-back and will likely cost considerably more. They will likely be a larger diameter as well, which will increase drone considerably, and possibly create that "bog", or shift the torque curve higher into the RPM band, like you said you wanted to avoid.