M135i exhaust reccoomendations

[Stupeo]

So, I've never been in a x35i let alone driven one. But that has not stopped me from putting a deposit down on a new one that should be here next week!

After watching youtube, the stock exhaust sounds good to my ears (my previous three cars have been 520d's), but I want something with a great sound - any performance benefits would be a plus.

Any recommendations? I've never had to worry about this before

Finally, I don't want to void warrranty, are there any manufactures that BMW approve of?

Thanks!