Loud?
Hmmm...
Sound is a very subjective matter; what is "loud" to one person may seem "quiet" to another. I suggest you find a fellow 03/04 cobra owner who has the exahust combo you are interested in, and go for a short drive together - listen to it in person, it's the best way!
Further, the Magnaflow mufflers are on the "quieter" end of the spectrum IMHO, especially if you still run the factory H pipe with the Magnaflow catback (or even a catted aftermarket midpipe - it's still relatively tame). Again, this is all subjective, but I think most would agree.
I don't think adding the Magnaflow catback to your cobra would make things "too loud." Of course, it's all relative.
Additionally, all catback systems will "drone" if you hold the throttle at that "sweet spot." I'm not an expert in harmonics or resonance, but we can tell when we are droning along at that certain speed/RPM, right? LOL
Generally speaking, straight through muffler designs (such as Magnaflow or Bassani) tend to drone less than their chambered muffler counterparts (such as MAC or
Flowmaster).
G'luck!