I have an Apexi ECV welded inline with my exhaust, so not the flange style
The difference in noise is heaps, if you pull it up and drive with it on it sounds like a stock car with a standard exhaust
Problem is the power restriction - it restricts the power heaps, I could barely make 5psi with the ECV activated where as when it was open it would make an easy 13psi
The idea of it is to restrict the exhaust making it quieter but its not to be driven hard with it activated, so don't boost it or try and push the car hard as it won't go and could cause damage
I'm sure Apexi advertise that it quietens down to 10db - I think that’s what it says on the box if I remember correctly