Looks like M4 has a full system and a slip on available now. I found it scrolling through eBay and then went to their site. It’s probably the best looking so far(my opinion of course). $674 for the full system on eBay is pretty tempting.
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/253478557812
M4 Exhaust · High Performance Motorcycle Exhaust Systems
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/253478557812
M4 Exhaust · High Performance Motorcycle Exhaust Systems