My original plan was to do the same. It really doesn't matter which test pipe you use as you will be modifying it. I didn't want to buy catless pipes due to storing a full x pipe is very cumbersome.
Your best bet is welding VBands on the test pipes, cats, and rear exhaust part. This requires 6 Vbands, 2 Vband clamps.
MotorIQ had a nice article on how they did it.
https://motoiq.com/project-e90-m3-pa...xhaust-tips/3/
