Looking into getting a downpipe for my '15 MCS with JCW Pro Kit exhaust. Has anyone done this? I want to make sure that there isn't a drone when I'm on the highway, because I know going catless makes quite the difference.
With the valve closed, the exhaust still goes through a muffler, however I can't seem to find anyone's experience with this setup.

 

Thats the exact setup I have. No drone. You will need a tune to keep from throwing a CEL though, at least with the Milltek. I have more details and pictures on the thread linked in my signature.

 

The U.S. JCW pro exhaust still has a secondary cat in it. So if you want to be full catless you will need to get the Pro exhaust modified.

Also its a 50/50 shot of your car not throwing a CEL for the primary cat missing. If you have a milltek that is your best chance to not get a CEL because they pull the secondary o2 out of the exhaust path. Of the 4 local f56's that I know with the milltek downpipe 1 of the 4 got a CEL. The other 3 have been CEL free for 10k+ miles each.

 

I think its a resonator in the exhaust, not a cat..... i think

 

it's an NGK cat

 

Of the four you know, are any of them tuned? My understanding is that to get rid of the code it has to be cleared by a tune, i'm still getting familiar with how it all works together though so i'm not sure. Would it be possible to clear the code using an OBD2 scanner (iCar2)? I assume the code would just show up every time the car notices the O2 levels are different than with cat.

 

all 4 are FBO cars. 3 of the 4 have JB4 and the one who got the CEL with the downpipe ended up selling the JB4 and got a flash tune.

In order to permanently disable emissions readiness you need to get a flash tune. IIRC bmw checks cat readiness after 8 drive cycles. I am not sure of the exact miles but I believe it is 200-300 miles it will check emissions readiness.
You could use a OBD scanner to reset the ECU every time the code pops up but personally I would find that annoying.

I assume the code would just show up every time the car notices the O2 levels are different than with cat
Correct. The ecu will check the voltage of the secondary o2 to verify the cat efficiency. If the voltage of the secondary o2 is too close to the primary o2 the ECU will trigger a check engine light.