I guess it's my mileage on my car then, I tried all that and a bag of chips. I never actually removed the clamps, I figured (as you said) that they are permanently part of the pipe.
Out of the three important ones, I was able to get none of them (neither of the two joining at the center muffler, nor the one on the p/s downpipe). The one on the very back by the muffler came loose with some good heat/twisting/etc. but I merely did this one for practice.
The Chrysler tech that I work with said yes they are a bitch at higher mileage, that heat is the best way to get them off but they are very often cut off. There isn't a huge amount of corrosion the pipes are fairly clean, but you can imagine at 282,000+ they are together pretty well.
I guess for my cat-back, I'll be cutting them off. :( I was hoping to keep the stock exhaust just incase something went wrong and it sounded bad.