Do you intend to leave the cat in place or are you going to remove that as well?
I have a Nur Spec R on my STi (plus I still have the cat in place) and its definately loud, but I don't think its stupid. A Nur Spec R on a cat-less car is
noisy though!
By way of comparison my car had a Cobra Sport system on it when I bought it and I got rid of that because it used to drone really badly on the motorway (plus I thought it looked like a giant bean can!). I swapped that for a titanium STi muffler (which I think was around about the same volume as the Milltek) and that was much quieter: you could only really hear anything in the cabin above 4000 RPM...
...but I missed hearing the burble all the time, so I ended up with a Nur Spec R!
I spend ages debating what to get before choosing the Nur; I watched loads of videos on You Tube (including the one Steve PPP posted!), but it was going to meets and listening to cars that I found was the best judge.