I have no experience with turbo exhausts so we're looking for you guys to tell us. I believe they are slightly different, which means you would require the entire section between the turbo and the catback.
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.
Neoshi is correct. The stock downpipe is made up of 2 pieces. Some time in '06 the front piece got longer and the back piece got shorter, or the other way around. If you get the "complete" downpipe -- both pieces bolted together -- it will not matter. If you are looking to keep one section but replace the cat section, it will matter.
The bellmouth on the stock downpipe is very restrictive. You are much better off buying a cheapo used catless aftermarket pipe, you should be able to pick one up for under $100.
I should say, that's how it went with the Impreza downpipes. I am assuming the same happened to '06-'08 FXT since our engine is same as '06-'07 WRX, but I could be wrong, Subaru does weird stuff with the Forester. If someone has their stock '04/'05 downpipe and is willing to measure the 2 sections, I'll measure my '07 stocker.
I'm going to post even more from now on just so I can look at my April Fools avatar.
Just let me know what you want to do. I can always separate the two parts (this means another gasket will be required). But the weight will be increased of course, so that'll mean shipping goes up. Good news is I have a box though. Padding material is another story.
I just did a COBB catted DP install on my FXT 04. You won't understand what a piece of crap the stock DP is until you set them next to each other. The COBB's mouth is HUGE compared to the 2.5" opening on the stock one.
I know it's expensive. I fought like hell to go the cheap route but it won't yield the results you're looking for.
If you're going to keep being cheap, find another hobby; otherwise, save up some cash and buy a aftermarket downpipe. Don't bother with cat-back, the gains are minimal and definitely not worth the money unless you're looking for louder exhaust.
Does anybody know the length of the OEM 07 midpipe? (centre flange to centre flange) I installed my DP and it appears to be a couple of inches longer, everything bolted on but the hangers don't match up.