Yeah that's why I prefer his phrasing in X-Men Black Sun..Jean says she gained Psylocke's "specialized" telepathic abilities...not her strength. Which explains the Phoenix raptor "sum totality" attack, and the Dark Astral form/eye emblem....but I hated that it had no tangible purpose...to me it should have made her completely invisible on the psi-plane or to psychic scans/detection, but nope.
To be far, Jean says "greatest" not most powerful, which to me would seem to encompass his high level of skills and experience, along with his power level...also Jean is humble and holds him in high esteem so of course she would refer to him that way, not to mention it's meant to invoke the scope of danger Nova represents