There are several proposals for improvements to the traditional unpack, and a significant one for a new design. You ought to read that RFC.

My point is that if it's moved out of the core, why be limited to just one "official" one?

Also, I don't think adding user-defined types to traditional unpack would slow it down one iota. Consider: internally it switches on the letter and jumps to suitable code for that. Adding more entries for more letters will have no effect on the stuff that's already there.

—John