The reason why I say not to change out the catback is because it is a special catback used for only the Bullitt. Since you are looking for the muscle car sound I would suggest a O/R H pipe first and if you arent satisfied with that sound ( I personally think it sounds exactly like a 60s musclecar with the o/r h pipe). Then add the Borla exhaust. But dont let the shop cut up ur Bullitt catback as you can sell if for a pretty good price.
IDk if you know this, but it's not used only for the bullit.;-)