So, I couldn't find a reasonable price for just a connecting pipe from the other brands. I received quotes ranging from $290-$600 not including shipping. I found a local shop in Houston called Afordable Muffler who built a custom connecting pipe. Thankfully, they had an old flange that was basically the same size, but needed some tweeking. Here are some pics and videos.
Now for my review. The Dinan sounds great. It's louder, but not abnoxious. You can hear it across the full RPM range unlike the OEM which was my biggest complaint. Drone appears to be well controlled. I haven't driven on smooth asphalt yet, so I can't say for sure how well controlled it is. The only dislikes so far is that it hangs a little low and is quite visible. Thanks to BMW of North Houston for handling my Dinan order.