Hi there, congrats on your new purchase.
I’ll chuck in my 2p worth. If only changing the back box is what you are looking at, don’t focus on LOUDEST, but nicest sounding instead. I have a feeling that the Monza is the loudest, but I wouldn’t get a valved exhaust as they are prone to sticking and thus failing. Also, in my opinion, a rear diffuser cut for two big pipes is meant for two big pipes, not two double pea shooters.
So, again, you do have many options, but my opinion is that the MM Bombardone is probably you best, most cost effective, straight through design backbox, which as I understand it, correct if I’m wrong, will give you a lovely crackle n grumble. Don’t risk making it sound too chavvy, I like the stock one tbh, but I wouldn’t say no to a Bombardone.

Definitely get a tune, I got a Quantum tune on my 2020 Turismo, made a big difference. Loads more torque. If you are going to get the back box, and whatever else takes your fancy, then a custom tune is better I would say.

I hope others out there can also give advice, especially if they have more experience with different exhausts and tunes. I’ve heard Tune4performance is good, a guy there called Tony. Also look at ******.

Good luck man.

Hi, another 2p worth, had a Novitec fitted to our comp, two and a half inch straight through system with four inch tail pipes, comes in three pieces so you can align it in the rear panel, sounded superb and the car got loads of comments on the sound.