Well, the thing is, Diana as Byrne saw her was already at minor god level. Perez thought of her as a mortal; she was strong, she could fly, she was good with animals, but she wasn't on the level of Phobos and Deimos. It was her tools that made up the difference - the lasso, the bracers, the talisman of Harmonia, these outside forces that she wielded that made her competitive with the gods.
But to Byrne's way of thinking, Diana should be cool because she's Diana, not because she has a shiny rope that anyone could use, so he made Diana's advantages intrinsic, made the lasso simply a channel and focus for Diana's inherent power, the bracers just a backup for Barda-level endurance. And at that point, Diana *is* on the level of a minor god already. Minor gods have weird intrinsic powers beyond the mere physical - Deimos' snake-beard business, the ability to sway men's minds to obedience through fear - but standard demigod Diana has intrinsic powers on that level as well (what she can do with the lasso is no less than what Deimos could do with his snakes, and she provokes honesty and awe in those around her the same way Deimos provokes fear). So... if you promote a demigod who's already on the level of a minor god, you're promoting her to the majors, really, because where else is there to go?
Which means, yeah, Byrne pretty much made her level with the twelve Olympians, as you can see in bits here - the being-in-three-places-at-once to solve the problems Athena showed her, the preserving of Polly and Wally's memories of Donna and provoking Donna's understanding of her power against Dark Angel while she's running around Hell - if she's not on Athena's level, she's at the very least on Circe's, which is well above where you generally want a character who's leading an ongoing. (Though I dunno, I bet Giffen on "Mxy: the Series" could be pretty awesome...)
has Diana ever been refered to as "Godslayer?"
It's... possible Diana's been referred to as "Godslayer," but it's also quite possible I'm thinking of Xena (which would be a decent reason to not see it in much use now). Personally, I'd prefer to get "once-princess once-goddess" back, but fuck that, right? Diana as a princess is iconic. -_-