Finally, I deleted the secondary cats of my xdrive car.
I bought this car and I had always find that the exhaust note of a 335i (n55) is louder than my 335xdrive (n55).
So I do some search and find on e90post forum that 335xdrive 2011 cars have secondary cats.
I find this thread : http://www.m3post.com/forums/showthread.php?t=457202
...And I did like this.
I only cut the exhaust and remove the catalyst material and reinstall the exhaust.
The result is good. The sound is a little more sportier and louder .... like a 335i 2011 with no secondary cat.
I don't know if I had an hp gain, but I have not find that I lose anything after one day of driving....
Took me about 2 hours for uninstall and reinstall and 30$ at a shop for welding.