Hero-deities and quasi-deities are about on the power of the Seven Sisters and the Chosen of the Gods of Faerun, like Elminister, at best. Azuth is a former mortal, as is Savros, who competed with him for godhood as the Lord of all Spells. So were Bane, Baal, Myrkul, Cyric, Kelemvor, Torm, Malar, Gwaeron Windstrom, and Finder, and they got further up the godhood ladder.Like I said, you're not gonna find that many high level mortal spellcasters on Oreth, but there's a crapton of gods who were formerly mortals. Zagyg, Vecna, Murlynd, Quaal, Heward, Keoghtom, Nolzur, Daern the Unshakable (not to be confused with Daern of Hawksroot), and Iuz
Kelenan's AD&D statblock is blown away by Elminister's, as is Murlynd's.
So, mages becoming very low-powered quasi-deities, which if you've read the original stats for the like of Murlynd and Kelenan, they are, isn't an argument, either.
They certainly have more flavor and give Oerth a very different character then Faerun... but it's a gritty character, not a hyped up powerful one.