These downpipes seem stupid overpriced.
https://x-ph.com/evolution-racewerks-f90-m5-downpipes/
These are one of the better priced downpipes I've come across; $715.50.
Also came across Megan Racing lower downpipes for $550 but haven't been able to find any information about them.
https://shop.redline360.com/products...9-mr-ssdp-bf90
Another options I'm considering -open up the mufflers and straight pipe the exhaust through the mufflers. I had this done on my 2011 M3. Haven't come across anyone on the forum that's done it though.