Maybe not responsive to OP's question, but I think the flex section in each part of the TurboXS downipipe and the race pipe are very beneficial. They ease installation considerably in lining up everything. They help make sure that when you are tightening things up that the gaskets and flanges are pulled together without producing any leaks. Also, they help isolate the exhaust and reduce intrusion of vibrations into the cabin of the car.
Now, I'm running catless, but there's no reason not to put a high flow cat in the racepipe, or to run the stock midpipe with its cat, if you need to do so. Or at least swap to it come emission testing time.
Let me say this: The restrictions in the exhaust system are significant. They are not, as we know, significant in the CBE. The problems are higher up -- the stock downpipe's pancake shaped chamber and sharp angles are very restrictive, and those two low flow cats are too. By the time the exhaust gasses get to the CBE they have cooled down enough that the reduction in pipe diameter does not produce any meaningful reduction in flow. But a 3 inch pipe closer to the turbo is very helpful. Also, because of the shape of the K04 turbo's turbine discharge and the wastegate suggest that a big bell mouth shape for the initial part of the downipipe will help reduce turbulence and, therefore, backpressure on the system at that critical point. Look for a downipipe with a good open bell mouth shape and a more gradual curve as it turns downward toward the midpipe section.