If you want a quiet exhaust--buy a Rolls Royce!
Someone is onto something in regards to the undercoating.Not only does it "protect" is also deadens any noise from the ehxaust.
But not everybody likes the look of its black ugliness-especially after all the hard work detailing the belly of the beast.
May I suggest installing dynamat or something similar under the carpet to quiet the cabin.
Nobody sees it,and you can still show the underside nice and purdy.Then run whatever exhaust you prefer.
Much easier and affordable than spending big bux and wasting time on swapping exhaust systems that will still be loud.