More information, please. Give us a link to exactly which components you are asking about.
I have seen some aftermarket turbo systems (not necessarily for this car) that use the stock exhaust manifold and catalyst, and drive the turbo with the exhaust from downstream of the catalyst. It's not an ideal scenario, but on cars with a tight engine compartment it can get the turbo out of there. It also has the advantage that if emissions compliance is a concern, you are retaining the stock catalyst in the stock position.