A 5 inch cat back want help. With a cat back all you are replacing is the pipe from the cat convertor out the rear, you will still have 4 inch pipes from the turbo and then there is the cat itself. All you need is a test pipe from DPPI and the MBRP spiral flow muffler.
I took my AFE off and put the stock air box back on and I am now using one of the new Amsoil Ea Air filters. It would be to your advantage to either buy an AFE torque tube or do some research and open up the tube between the air box and the turbo.
Where do you live? I have a stock tube that has been opend up if you want it I will ship it to you for shipping.