You will change the sound when changing the muffler, but you can't gain any power by modifying any factory exhaust system part.
Contrary to many who believe it does gain power, the math just isn't on their side, like Cold Air Intakes(CAI) boost power, they don't on any fuel injected vehicle.
In the "old days" with carbs, CAI did work because they weren't installed at the factory.
And in the "old days" scavenging exhausts(tuned exhaust) were not installed at the factory, so tuned exhausts DID add power.
But car makers learn from the hot rodders, CAI and tuned exhausts have been standard features on all engines since mid-80's.
Your 3.0l exhaust(headers) are tuned for best mid-range RPM performance, you can change to low-RPM range headers, loosing the mid-range but gaining better off the line performance, you don't gain power, just change when the power is available in the RPM range, you can also get racing header that change it to a high RPM range
Muffler, as long as pipe diameter is equal or larger, won't effect performance in any way, but it can sound better.
And cat converters do NOT effect performance either, just FYI, another popular myth, lol.
If cat converter was clogged then yes, but working cat doesn't effect anything but cleaner air.