Once that exh gases cool around the rear end, you can decrease the size of the tailpipes with little effect on HP. It will also quieten the car some and the further rearwards and the bigger the mufflers are the quieter it will be as well.

My 572 Hemi, runs 3.5" collector into a 3.5" in/out X pipe, after the X it tapers nicely to 3" at the rear end and 3" out the back with into large case 3" Magnaflows at the very rear of the car. Fuel tank is in the trunk so have the space.

Car is pretty quiet with 11:1 CR and over .700" lift roller.