Did we ever actually get to see Darla fight? In Season 1 of Buffy, she used guns before finally being dusted, but I can't remember anything beyond that. Still, we never saw Dru fight, either, but she managed, off screen, to wipe out a train full of people, so I guess you can't sell the lady vamps short.
My guess would be that ensouled vampire Darla would be written as being not quite as powerful as Faith, but possibly more formidable, due to her centuries long experience. She just might give Angelus a run for his money, especially if it were established that she was the one who actually taught him to fight in the first place. It might be a case of, "You know everything that I taught you, but that doesn't mean that you know everything that I know."