Hey guys i know alot of people are actually looking up for this in the forum cox i am one of them. So i did some research and calling and here is what i found so far.
So as far as the quad exhaust for a f10 528i goes, ur CHOICES are nt many. but still a few is availble.
FIRST thing first, most Quad exhaust needs a different diffuser, meaning you have to BUY another diffuser for it and that is usually $1k and up.
SECONDLY, you have to have the M-sport bumper. Which is abt
$880 without the M difduser. That is excluding the paint job. There are some diffuser that works on a regular non M bumper but trust me its nt that nice looking IMO.
LASTLY, the Quad exhaust. There are a few really cool ones out there:
1. Hamann
2. Supersprint
3. Arqray ( my fave n im getting them, they are $2650~ for the Ti tips ) 
4. Eisenmann ( $1400~ ( pounds ) i think they just release them, idk never seen a video of it yet. )
5. Royal Mufflers ( these requires you to bring them n let them customized, USA )
tts abt it currently but they are SICK!!
For diffuser wise:
1. 3D design Carbon FIber ($1600~)
2. Hamann ( but you need to install the spoiler too )
3. End.CC Carbon Fiber ( $1800~ ), [ these are the best Looking IMO right now. ]
4. AC Shintzer (although idk if the round tips fit )
5. Kelleners
6. Lumma ( still not out yet but prepare the $$$$ if you want them ) 
7. VORSTEINER!! ( omg i love this and they are due to release this summer, its the reason im waiting for to get my exhaust ) 
So yea here is a list of stuff that i researched out these couple of weeks.. It's pureply based on my opinion so please RESPECT it. Also i think some of them fits the 535i too. so check them out. I hope it helps some people cox i took a whole lot of time finding and calling the distributor for the price and all.
ENJOY!!
Feel free to comment and add on anything that i missed so i get to know more.. Im currently just waiting for the Vorsteiner's diffuser.