My rec room is definitely more lively than other rooms in my house. No question about that. And yes, the clap test does indeed reveal this. However, I don't think it's too lively as evidenced by my reverb time below.
Regarding the treatment of the first reflection points, I agree that the ceiling and floors should definitely be treated. And perhaps the laterals need to be treated for some rooms but the question is "how much treatment?". I say perhaps because if you have your speakers toed in "severely" and they are far enough from the side walls, then you may not have anything to worry about.
The lateral reflections give rise to a sense of spatiousness. Of course if they are too live, that's not good either. In my room, with the amount of toe-in from the fronts and the distances to the side-walls, I calculated that I am getting the right amount of spatiousness. And my ears tell me so too. The ceiling area between me and my M80s however is another matter and I need to take care of that right away.
Come to think of it, if you listen to all of your audio in multi-channel, you may want to add "a lot" of treatment to your room since you don't need to rely on any reflections for ambience.
After I finish this project, I'll be able to write a thesis on room acoustics
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