Where do you get the idea the EO don't go as far as the RCC? Orthodoxy holds to the sinlessness of Mary, I don't think any of the EO churches teach otherwise.

Ephesus 431 was answering some questions about Christ's nature, somewhat indirectly. The council determined it was inappropriate to deny the title of Theotokos to Mary. Nestorius preferred the title Christotokos and had some limited reservations about the title Theotokos. The council didn't name her that, it upheld that it was heretical to deny her that title, which was already in use.