Mui Chiu Fung was the most proficient user of 9 Yum White Bone Claws in LOCH. After the death of her husband, nobody was able to use this skill better than she could (although the only other user was Yeung Hong). Her proficiency in this skill was good enough to terrorize the likes of Au Yeung Hak, the Gong Nam 7 Freaks, and the *individual* Cheun Jen 7 Disciples, but posed no threat at all to Greats-level fighters.
After Chow Chi Yerk learned the 9 Yum White Bone Claws in HSDS, she too was not really a match for any Greats-level fighter, but seemed able to stymie the sub-Greats of her era (e.g. the Mo Dong Heroes, Ming Cult Guardian Lords, etc.) in a way that Mui Chiu Fung could not do so decisively. Would you say that Chow Chi Yerk's mastery of 9 Yum White Bone Claws surpassed that of Mui Chiu Fung?