I've actually been doing some research for a quad exhaust myself. However I'm not driving F10 but an E60 and I believe it would be somewhat the same
Point 1:
There are properly designed quad exhausts in the market from reputable brands like Remus, Hamann, Eisenmann, SuperSprint, etc.
Pros: They are properly tuned, guaranteed to give you performance (if it's stated in the specifications), plug and play, comes with warranty, beautiful sound.
Cons: Costs an arm and a leg
Point 2:
If we go to local exhaust shops, most of them can do a customized version of quad exhaust to be retrofitted into our rides.
Pros: CHEAP, costs almost 3x - 4x cheaper from branded ones depending to which ones you are comparing against.
Cons: The performance is not guaranteed and in some cases, you will get a drop in low end torque, drone, vibration, doesn't sound good, etc. The reason is because they are using some standard rear mufflers that is universal for all sorts of cars and it's not properly optimized for our model. Although they can retrofit pretty well and looks good, these are the risks to take
