UMan -
I didn't see any mention of making them thicker just for MIG'ing. I guess that could be a reason since they mostly support folks homebrewing lawnchair bikes, trikes, trailers, etc. Their site has some impressive photos of completely projects.
I guess we could ask Frank/Welder if MIG actually puts more heat into the weld that TIG. He would know. I have both tools, but never compared them... but I have never MIG'd anything for a bike either for that matter. I had always suspected that a TIG provided a stronger joint, but I have built frames for my street rods with the MIG and they never broke a joint...so maybe I am just muddled on that point. The availability of rod in Chromoly for the TIG is helpful for bike building.
BTW; Yes, I have used Paragon shells and much of their other build bits; it is good stuff. For the shells for some special applications, I just wanted a bit more meat on the bone and looked/found the Zombie ones. I doubt either would break even under extreme use.
/Kerry