Unless you remove the GPF's the exhaust is never going to be very loud, nothing wrong with silent but deadly
As for the mudflaps on an SQ2, I have them on mine. Took a lot of effort because whilst there are many adverts on FleaBay claiming to have mudflaps that fit an SQ2, the reality of this is not the same.
I fitted the fronts OK & but had to contact the seller "in China" about the rear and they sent me another set of rears.
They were not right either so I sent loads of photo's over and they did some digging and then sent me a picture of a rear mudflap of what they thought might fit. (unfortunately I do not know what they were "meant" to fit) These did not fit right either but were close, the seller refunded half my money back, I did use the fronts OK. The seller did a great job trying to sort this out.
Before I binned them I decide to have a "go" at the closest fitting rears with a scalpel. I got them to fit 99.9% perfect, still way better than most I see fitted to other cars..
So "yes" you can have mudflaps on the fronts easy, the rears not so easy.