I have a purchased a high-flow cat for the purpose of creating my own exhaust based on the MattSpec design. However, after looking around and talking to shops, I think I would rather buy a pre-made downpipe to eliminate the toughest bends and have a bellmouth (helix) openning from the turbo. Is it possible, or a good idea, to add the high-flow cat and muffler after the downpipe section (this new section would be created by a shop of course)?

 

That's what I did.
I used a Bosal down-pipe then made a mid-pipe using a 3" hi-flow cat and bolted it to a custom 3" catback exhaust.
It works fine and saved me some cash.

 

Ooops, I see what your saying. You want to install it past the 3rd cat point. You may be putting the cat too far back to get optimum performance out of it. I don't know of anyone who has installed one that far back, so you would be blazing new trails.
I would recomend a Bosal, Stromung, Scoobysport or even a TurboXS dp then make a mid pipe and have it mate up to your cat-back.

 

I really like the Bosal dp but it exists at 2.5" and I would rather have a 3" exhaust. I think Stromung's dp is 3" out, but I am not sure I like the design so much. Since the shop is making custom stuff for me anyway, maybe I can just shorten the Helix dp and add the cat where the 2nd cat would be.

 

Take a look at the turboxs one. I got one, and i thought it was a beauty when i pulled it out of the box. That way, you can still put on a cat in place of the third cat.