@ghoul It's nice that you value our opinions.
I don't think I've ever used "dead serious" and certainly not the other one. Why is that? Well, it's usually quite obvious when you're serious. Therefore, there's no reason to say that. Also, "dead-on serious" is, going by the small sample of usages I've seen, used to contrast with "humorous".
I don't disagree with Jutfrank. (It's not a contest.)