I suppose the key to these NPCs strengths is that they are all single class. Some single class good & neutral NPCs are fairly powerful too - eg Minsc, Valygar, Mazzy, but there aren't any good single class mages or clerics, so no comparison can be made with the evil ones. I suspect they would be as strong as Edwin & Viconia if they existed!
The nearest to a single class good mage is Nalia as she only dualled at level 4 and IMHO she rocks (if you can put up with the whining). She doesn't get as many spells as Edwin because she is not a specialist mage & doesn't have his amulet. But she is still powerful - kit her out with robe of Vecna and bracers AC3 and she will be one of your most useful characters.
------------------
Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas