Very good question?Unless one opens the mufflers to check the insides,it will be difficult to tell?I had this dilema when I decided to fit the 4.8is mufflers on my x5!
In your case ,if your thinking of fitting the 4.4 mufflers on your 4.8is,you need to confirm the actual exhaust pipe diameter?I know the 4.8is has 63mm diameter and I believe the 4.4 is inferior!So ,irrespective of whether or not the 4.8is have more flow,if you fit a smaller diameter pipe ..it will be a restriction...