There are many fast-and-easy ways of exhaust conversion by changing/adding the tips only and leaving the whole OEM exhaust (maybe also a stock diffuser) almost unaffected. But you may decide to go for a bigger work replacing either axleback or even the complete catback with a favorite retrofit (also diffuser), and in my case it was Supersprint and Vorsteiner Aero M5. To complete such a project you will need to apply much more effort and take higher cost, but the result will provide your car with better performance (and sound) characteristics, a cooler look and feel and therefore you will have much more fun.
The most popular aftermarket diffuser for the 5-series is the M5-type. At the pictures below you can see a comparison of two diffusers: one is designed for the 5-series and another one is for the M5. They look and fit the quad tips the very different ways:
Important note: the M5 tips are located closer to the center, about one inch compared to the 5-series.
Because of much better aesthetics of the M5 diffuser and more accurate fit to the quads I decided to go with it. So here is my Vorsteiner Aero M5 set delivered by the local distributor:
With the Supersprint that has (at my opinion) the best website between all the other vendors, there is a number of configurations of the center part and mufflers:
http://www.supersprint.com/en/IT/prodt-5-7051-1.aspx. There are two options with Supersprint: with "normal" quad tips and the ones "for M5 rear bumper" (part numbers shown for my 530d):
I have chosen the normal or classic Supersprint mufflers and tips because a) when I've been ordering the exhaust, the M5 style was not commercially available yet, and b) honestly I don't like what they've done to the tips shifting them towards center:
So the exhaust system conversion on my car was done with the selected normal Supersprint quad variant. The tech was a very professional guy so he has adjusted the pipes exactly the necessary way to make the tips fitting the M5 diffuser:
And I believe, the whole work is done very well and the final look is perfect:
