My other car is a MINI!!!!
I've heard a few ko4's with decats sounding like this, although always when using standard catback system.
On my daily I ran a downpipe and decat with the standard catback, and it sounded tinny and ****.
On my track car I ran a downpipe and decat into a milltek system, and it sounded great, not at all tinny.
I now have a cheap aftermarket catback on the daily, and it still sounds a bit tinny, but not SO bad.
Will update with how the ko4 sounds with the XS downpipe when I get it running! Although it does seem to be a feature of highly free flowing systems that they get a bit raspy.