It is heavily implied he is. "Although his true strength has yet to be seen, Wonderweiss is shown to have strength on level with an Espada, being counted as one by the Soul Society, and being sent on mission with others. The fact that he could sneak up on Jūshirō Ukitake, one of Soul Society's most capable and powerful captains, and mortally wound him with one strike and destroy Hitsugaya's huge ice obelisk with a simple cry gives hint of his actual power. Even Starrk, the Primera Espada, said if Wonderweiss is in battle, Aizen is wanting to end the fight, further hinting on the Arrancar's powers."