[supercati]

I did this to my Euro 130i (E81).

Here in Europe we don't have the secondary cats, so I went resonator delete (modified the stock one) + 135i N54 muffler / backbox.

Sound is really really great. Pops/crackle when releasing throttle, plus louder sound when acceleration. Overall, it just sound louder and a little bit more agressive, and on the other hand, if you just cruise around in a normal way, is still acceptable, almost no drone at highway, etc.

If I were you, I'd definitely go for: secondary cats delete, resonator delete (modified or YPipe), 135i N54/N55 muffler.
You could also use 135i N55 resonator and modify it to be like the BMW Performance (easy), and it will be like an Y Pipe.