Holy hormones batman....
I would say there is def a reason 95% of all exhausts have flex sections, or utilize the donut style gasket connection. It really does have to be able to move around. Stock STI up pipes use a flex section, and I've personally seen one of those break over time (just ask Mark). Also ask anyone that's used a straight (hard) up pipe. Most have issues with it not sealing good over time. They develop leaks at the gaskets.
I really want to rid myself of this mess I have going on with the connection on my 3" DP to my SPT Catback (donut gasket). I have this silly adapter with a 3" gasket, and then a donut gasket on the other side, all being held together with longer (napa brand) spring bolts. I would love to add a 3" bolt up flange to ditch the mess I have on my DP now, but the only way I would do this is having a flex section added to the midpipe near the new flange.