I've got a Milltek non-res cat back on mine - had it for 3 years this coming October. I had the same worries on mine about it being too loud, but in my opinion it gives the car just the right amount of noise / bark. Still unintrusive when cruising along, but just a nicer deeper sound when pressing on.
No issues with fitment or anything. The stainless tips polish up perfectly every time, and are much better then OEM.
Bought mine from AwesomeGTI and then was fitted by APS a couple of months later. Only did it that way so I could have a few jobs done on the same workshop visit, and so I felt like I've staggered the spend a bit!