I don't know anything about BCS, but what I can tell you is the Miltek system sounds really nice, but even the non-resonated system I have isn't in-your-face loud. You notice it and at higher revs it makes a great noise, but when you settle down for motorway duties or drive gently, it disappears into the background.
If you're after more noise though, don't forget about the downpipe and sports cat options too, as these will aid a better sound too... I'm only just about to fully enjoy my new level of power and sound but I'm already looking at going stage 2 on it!
It's a bug, once it's bitten you're as good as gone...