You will have a hiss at idle due to the vane position of the turbo until you tune it out with EFI or the newest Edge. Some people don't mind the hiss. I don't like it. I am running the 4" MBRP downpipe back with a 4 inch Magnaflow clamped in cat. Even with the cat the hiss is there.
5" will not provide you any added gain over the 4". Just a deeper tone. The 4" with no muffler no cat is plenty loud. Maybe too loud depending on your personal preference. That is why I would order it with the muffler and you can delete it later if it isn't loud enough for you.
Just a suggestion, you may want to put some details about your truck into your signature. Other members will be able to assist you better with that info.