Do you know how much hp/tq over FRPP magnaflows offer on a mostly stock 4.6 mustang to be worth a possibility of getting a (little) drone? His main objective it to keep it mostly stock sounding, and induce no interior noise. I have seen many people post on here that magnaflow's catbacks do drone atleast to some degree. In my experience, what someone's defination of acceptable drone is a personal choice, especially when it is on the small order. What might be OK for you might not be OK for someone else - especially if the car is driven at that rpm, which is typically hwy speeds on stock gears. I have never heard of anyone using the FRPP catback that has complained about drone.
My vote is for a SS Magnaflow X mid pipe as you gain power and sound without adding the possibility of drone from not changing out the stock catback, but that costs some bucks - even so, I think it's worth the wait.
I just put in a MAC Prochamber off-road H and love it. The total cost was :$334, including installation, shipping, Mils, etc. I got it off ebay brand new from a dealer for $204.99. By the way, you can get MIL eliminators off ebay for $34.99, half the price most places charge and they were brand new, perfect from a dealer. I kept the stock mufflers and catback, since the sound was exactly what I wanted - quiet inside the car, but with a much more throaty sound outside. It evewn lightly burbles at idle, all without being too loud. Under full throttle, it sounds awesome, in 5th on the interstate it's dead silent. The Mac Prochamber get's my vote. And the best part is you can purchase all the parts seperatly and do it over time if you need to.