OP I feel for you. I have a ds who sounds just like yours. I haven't always handled him well, and I've shouted and smacked sometimes which I'm not proud of. I also have 2 other dcs who are pretty easy to handle. Some dcs are just hard work. My ds is now 10 and could still kick off if he doesn't get things his way, although it is MUCH less frequent these days.
My ds just seems to feel everything to the extreme. When your ds is happy, is it really obvious? He wants control and power in every situation, and his mood dictates the atmosphere for the whole family. I've decided that I'm not taking any blame for the way he is, and also no credit for the improvements we've seen! However, I have found that a calm but firm and consistent approach has been the most effective. I also massively pick my battles. The book 123 Magic has been great - with the getting into the cat situation you described, I would use 123 with a consequence on 3 if he doesn't obey. It would depend on how much of a hurry I was in. Or I might wait until he asks for something he wants, then tell him "sure, you can have/do that as soon as you've done X"
We had an almost-tantrum this evening, just to give you an example of how things improve with these kind of kids as they mature and gain more control of their emotions. We went out for dinner with my parents. DS hardly ate any of his main course. I think he was wishing he'd ordered something different. After giving him a long time and warning him that he wasn't going to be ordering dessert if he didn't eat the main course (my parents paid so I was conscious of that too!) he decided to leave his food. Then he wanted dessert. His sisters had eaten well so they could order it, but I didn't let ds. There was a bit of a scene where he complained loudly and pretended to punch me (cue grandparents looking VERY awkward about people looking at us) then he ran outside to the playground. I think he knew he needed to calm himself down and that I would not give in. A couple of years ago that would have been a major meltdown lasting up to 2 hours. So although it would have been easier just to get him the dessert, I do think it's been the right decision to follow through, even though it's been so hard at times!
That was longer than planned, sorry!