The only real change between the 04 and the 08 XT turboback is the size of the midpipe resonator. Their might also be some heatshield changes, but length, size and bolt patterns are all the same.
An aftermarket catless downpipe is going to flow WAY better than a decatted OEM pipe. It is cheaper to decat a stock, but not really that much cheaper. A decatted downpipe will require a tune, and if your going to do all that you might as well do it right the first time and get a 3" aftermarket downpipe. Just having a nice big bellmount is a huge improvement over the tiny stock opening.