hi,
i have the same problem - i've got a full miltek system but too loud - but i don't know if changing the cat back would necessarily change the sound in the cabin that much - i've got a bbr cylinder head and i suspect that may also be contributing. i was thinking about lightweight sound shielding options....you'll get more money for yours catback on ebay if you do sell - wait for one of those 10p listing days and put the link on the mini forum classified's section, that'll generate the most interest in the shortest space of time. (removed description of Ebay fraud - AW) i may have done this with some seats i sold on ebay recently....
i was thinking about the jcw cat back my self but i'm not sure that would make much of a difference to the volume...