Arguments that Aran'gar couldn't have survived if he was weaker don't make sense, since Moghedien clearly did. None of the Forsaken seem to think Moghedien was especially weak in the OP. Which tells me the general range of the Forsaken is a little more spread out.

I also think the fact that Graendal can do all this either on her own or with a weak angreal proves that she is at the higher levels, and almost certainly equalled Sammael at least, as Moridin's PoV suggested. This also lends further credence to the theory that Lanfear was, for all intents and purposes, as strong as Rand and Ishamael.