The resonator silences the exhaust, so removing it alone for straight pipes gives a decent increase in noise. Couple its removal with secondary cat delete (normally for an X-pipe) then this will give a very noticeable increase in noise.
If you do a search on decats / exhausts / X-pipes you'll find the answers to most questions you may have.
You see, this is not how I know this to work. Resonator, as the name suggests, is for cancelling the drone. What silences the exhaust is back boxes.
One either replaces the resonator with an X or H pipe but if left in place with additional X pipe it will lead to the whole exhaust being out of tune.
What's even worse is adding a smaller diameter X or H pipe as it will introduce a restriction.
But happy to be re-educated if there's some scientific backup available...
They effectively have an H pipe up by the secondary cats, it's if you remove them you really do need an H or X added.
The joiner before secondary cats is a Mercedes thing. BMWs don't have it. Both come with a resonator where pipes join. When you remove it you need a x/h pipe.