If you don't like to hear the exhaust inside the car then my recommendation is to consider adding sound deadening insulation in the trunk and under the back seat to start with. Then possibly on the interior of the doors under the door liners. I believe that Dynamat material might be a good sound insulator.
^^^+1 The Mustang is one big tin can. I went with a company called second skin and achieved two things. The outside sound in the car was reduced and the shaker 1000 could be turned up substantially with more clarity. Everything in the cabin was much tighter. It was a lot of work but worth it. I used the Damplifier Pro and topped it with Luxury Liner Pro. The areas that are the most needed are the spare tire and trunk areas,under the console, rear seat and quarter panels The downside is that its is not cheap and it adds weight.
2011 Red Candy
/build 2/1/11/ 401A
MT with Redline MTL/ Brembos/355's/glass roof
Shaker 1000/Scoops/Clear bra/Steeda UCA and PH bar
Redline hood lifts/Bilsteins with Ford Racing P springs & axle bks
UPR catch can/whiteline kdt928/MGW shifter with Epco ball
Shaftmasters DS/BMR loop & relocation brackets
Nitto 555 G2's/Steeda jack rails