I've got a miltek catback system fitted. They're about 1200-1400 (I think) plus fitting, but they are a big improvement on stock, but aren't obtrusively loud. Darkside sell them.
I have to agree with boxer as even with a dpf delete and no back boxes there not as loud as a std S5.Are you sure it’s only the rear silencers that have been removed and not also the dpf? I have my rear silencers deleted and the increase in sound, although good, it is minimal