To be honest with these cars, once you're stage 2, exhaust gains aren't going to make massive differences, and then a custom vs miltek, you're entering similar threads of stock vs aftermarket air filters.
I had a miltek on my b7 and was good, but at that cost...the only thing that I was impressed with vs custom was the high flow sports cat.
If doing the exhaust this time, I'm just going for looks and a bit more noise over OEM, simple back box and tips will do that for me.
My two cents would be go custom and put the savings vs miltek towards stage 2
I agree, back box with custom tips and a bit more noise is where I am looking. I don't feel I need to chase a few hp with the S4 it goes well enough as it is, and there are better ways to achieve it methinks. Each to their own though