on the B8 S4, you can get away with making a shitty exhaust because frankly none of them help much as the supercharger makes the magic happen and overcomes mildly restrictive exhaust setups. Cats even.
on a naturally aspirated car such as an B67 S4 or B78 RS4/5, you are relying on the exhaust to help the efficiency of the system. Taking air into the motor to make power is hurt by in3fficient exhaust manifolds/headers, downpipes, cats and catback. It is also helped if you improve it.
An efficient exhaust can literally help draw more air into the motor than something less efficient.
So, with the Milltek on the RS4, they basically made a worse quality catback that is the same size as stock. It’s a complete and utter waste of money. It’s also poorly designed with jagged bends. It sounds nice, which is the only redeeming feature. On a B8 S4, if it sounds best, it’s probably the best catback to own. On a B7 Rs4, sound is important, but there are a couple of other far superior performing choices that also sound fantastic.
This was one of the first ever posts on this forum, 3 years ago. RS4 Milltek vs. RS4 stock.