I'm kind of ambivalent about stats for uber-powerful demons and gods, although Divine and Defeated (shameless plug here) and Dieties and Demigods hae shown that it can be done well. I guess with the new epics rules coming out and the ability to play dieties, such entities should be detailed as players may very well encounter them.
Now as for the status of arch-devils and demon princes, I see no reason these beings can't be quasi-dieties or even true dieties. I really like the idea of Asmodeus from the GtH (even if it was a bit insulting as an athiest), but it really didn't mesh too well with all the other creation mythologies out there. I would place Asmodeus at about the power of a lesser god, with the others being either demigods or quasi-dieties. Remember, they were powerful enough to banish Gargauth, the 10th lord of the nine, from hell and he is a demipower. It takes a very powerful being (at least a quasi-diety) to rule an entire level of a plane, not many gods are even up to the feat.
-Michael Gill