M22

Advise not to get Milltek. They blew it with catback for twins. Seen too many people too often had problems with to consider theirs for these cars. Even if there is lifetime warranty for original owner, changing few times exhaust with involved time to get replacement and extra work to change to fix something that should work properly in first place seems not what i'd ever wish.
Also advising not to choose exhausts from youtube vids and other people reviews. Liking or not specific sound and loudness levels is very subjective thing for later, and online video recordings of unknown recording levels and varied recording equipment & settings used also hardly is usable to decide if to buy or not. At most .. if for some specific exhaust there are many many users and all speaking positively and along the lines one expect from product. Best way is to hear specific exhaust bit combo live in person, in eg. some trackday or car meet (unfortunately due pandemia these are rare occasions nowadays) and decide from that if one likes or not.

Unfortunately mid pipe will not help much. The aftermarket catbacks design it to avoid the weird bend that leads to the axle back straight rather than through the side , giving it less muffler area in back box.

So its a flow thing , not a noise thing except resonator bit which you shouldn't cut out or you get drone.

If you get an aftermarket manifold, which will get rid of the first cat. That will substantially take your noise levels up. The other option is to get a cat back that is loud enough for you , like milltek or even a resonated cobra.

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Invidia Q300 is a great catback , if that is Road legal. Can't compare it with your brand as I have never heard that one.

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Hmm? Legal in what sense? What are requirements for catback to be legal in Spain? Having something like TUV certificate? Or having not higher then specific loudness level? Or to not touch in any way existing catalisators? If later, then all catbacks should be legal, as they by name & definition are further back from cats, cat-backs.

I read is kind of "too quiet", but yes it is road legal
The question is mainly if it worth to change the whole catback just because of the less restricted resonator. Because I'm using the oem midpipe resonator with the aftermarket axelback, which I guess it's not too different from other "legal" axelbacks
 
 
 
Yes, something like the TUV certification is needed. They call it also CE certified
For example, Milltek is CE certified but Cobra is not. Then my options are quite limited because I want to keep it legal...
Maybe I will just change the manifold and leave the catback like that. 
Btw many thanks for your answers