650i adding hiflow cat to aftermarket catless downpipe

[antmay]

Hi all,
I have a 2015 650i XDrive Gran Coupe. I am thinking to go Stage 2 BootMod3. However stage 2 requires an upgraded downpipes. My state of Ohio requires echeck. I have found the AR Design downpipe with high flow cat for over $1600.

I am thinking to buy the catless downpipe which is under $400 and weld a 200 cell cat converter to it. I might save around $600.

Has anyone done this before? and if so, what issues do you have to face?

Thanks,
Anthony