I suggest you to avoid armytrix. Dosent last long enough.
Try ac schnitzer + akrapovic sport cat (its listed as for m140i and 240i but fit pefect 540i)
Or remus + sport cat. (In price of armytrix you will get nic sound and much better throotle response).
Cat less stinks..
Local garage replaced inside (cheap crap fibre glass inside that melt and burn away.. ). On monday I will be testing it
Hope to it bring it back to its glory. Local garage uses diffrent stuff (been given lifetime warranty on that fill ).