Sounds like you removed the upper heat shield on the cat. I don't thimk you have any reason for concern but what's my opinion wirth if you run into a problem. Take a look at your oil temperature gauge and as long as it doesn't kick much higher then don't sweat it. I have ran the RT highflow cat which sits damned near touching the pan and my oil temperature rose a slight amount. If it were me and I was running the stock cat, then yeah I would probably leave it on there and use some clamps to keep it from rattling. I agree with your muffler guy, it really doesn't matter and honestly I wouldn't waste a technicians time with this.
Ford dealers usually place a metal hose clamp around it (both shields), tighten it up and call it a day. You should be able to find the right size hose clamps anywhere and it's a 5 minute job if you felt like taking it on yourself.
I know that I've heard that before. I had my coolant temp drop slightly on the guage and my oil temp rise slightly when I did watter wetter and the RT highflow. I haven't ever looked at how its wired but the gauges went opposite each (slightly) with both. Shit I thought it was somewhat legit [?|]