I can't compare it to a Viggen but it certainly is a lot louder than the standard 9000 Aero. I ran an Abbott cat back system for a year before fitting the downpipe to get the full benefit. I almost changed straight back again as it sounded so incredibly loud. Now that I'm used to it it justs sounds incredible.
A lovely burble on idle and a head turning roar on full chat. Mmmm nice.