Hi, also have HKS hyper muffler rear box, the sound is fab, also a reasonable power increase.
I saw a guy at tdi with a twin pipe set up he had bottomed it out becuase it was so low, also you could see were the arch had been cut out for it to fit in to the body kit, looked a bit chad. You would need a custom made rear skirt. The system did not sound as nice as the HKS( personal preference).