They had an n55 335i that was pretty much my exact car there with full Cobb bolt ons.
1. What sort of gains did you see from it (and with what tune)?
A little more restrictive than catless was the answer
2. How much louder is it than the stock DP?
It was in-between the stock DP and my VRSF Catless DP (he had Cobb exhaust but that didn't make a difference on cold start and low revs)
3. Any increased smell?
It smelled stronger than stock but not bad at all
4. Have you passed an emissions inspection with it (and in what state)?
Wouldn't give me a straight answer.