Stranger things have happened I guess but if the cat was plugged bad enough that it would cause the engine to overheat, there would be a very noticeable decrease in power. It would feel like it is bogging down throughout the rpm range and your exhaust manifold gaskets would probably be blown out.

I've worked as an auto mechanic for over 7 years and I have seen plugged cats on lots of cars, none of which were causing overheating problems. I've seen them being confused for tranny problems but never causing overheating. I'm not saying that it definitely is unrelated to your overheating but I can't see any way that it would have caused you to blow out a head gasket or crack a head while you could still drive it anywhere. What I'm trying to say is, if it's plugged bad enough to get it to overheat bad enough to crack a head or blow out a head gasket, you would have to be flooring it everywhere and barely moving down the road. Hills would pretty much be out of the question.