There are spots on the TTi stuff that they acknowledge should be welded, but they give you clamps to hold it together until you get to the muffler shop.
I always have those multi piece sections welded together anyway.
I wouldn't put anything on my car that wasn't mandrel bent. Even the OEMs are finally starting to figure that one out.
redfastback: How much of the time was spent getting the headers to line up with the rest of the exhaust? That was a serious PITA on mine, and then the X-pipe was too wide to match up to the adapters. I ended up ratchet strapping the front "legs" of the X to get it narrow enough.