Hi
Just adding my experience to the thread.
I've found my cupra too noisy, presumably due to the bigger tyres (than previous 17").
I stripped the boot and rear seat and clad with 2mm foil backed rubber stuff (like silent coat). I did notice this shift the noise to the front slightly so must have made some difference. I then tried to do a bit around the front floor and doors. I struggled to add much to the floor so didn't feel like this made much difference, so...
I then added some 40mm foam to the front doors. This made a noticeable difference to the stereo and now the road noise sounds more like it's coming from the rear again.
Therefore the work must make a difference but I'm still struggling with the noise so don't think it's made much of an absolute difference. Admittedly this is a DIY job with cheaper products but thought worth sharing.
My next job is to add some foam to the boot and rear doors just to give a more satisfying thud on closing. And grease the squeaky back door.
I had the front wheel arch panel liner off and note that the standard insulation is just white fluff, so suspect upgrading this to foam could
help a little. Not looked at the rears yet to see if anything could be done.
My other thought is whether the 'flicks' to the wheel arches in the cupra create more wind noise somehow?