We don't have a really good sense for how powerful the sublime ancestors are compared to the other White to ascended beings (because from our perspective they'e all batshit bonkers) but I'm inclined to think that the sublime ancestors are greatly more powerful than white realms and possibly stand out around the ascended spirits. From the Fisher and Serpent side story we see the difference between the Fisherman (a White considered ridiculously powerful) and one of his feats is to survive just one flick from the serpents tail.
Grandmother Serpent was not a Sublime back then, and Yao most likely wasn't a White at the time either
Evidenced by the fact she was slithering around sunbathing and hunting and whatnot
Sublime Ancestors are dormant and inactive by nature because they are a more physical equivalent to the ascension undertaken by those who become Great Spirits
The Sublime's spirit ascends, but they uniquely leave their physical body behind, which lies in dormant slumber
That's the reason why they just stick around in one place without moving: Celestial Dragon's body endlessly circles above the Peaks, Grandmother Serpent sleeps in Lake Hei, The Reveler lies beneath a waterfall, etc.
Sublime Ancestors
are Gods, in the same sense that Great Spirits are, they just left something behind when they ascended
Their sleeping body also acts as a backdoor and vessel that allows them back into the physical world in a far more easily
For a Great Spirit, manifesting a direct avatar with power beyond Violet level is a costly expenditure of resources, and coming back as a Physical God beyond White Level is nigh unheard of due to how costly it'd be
(Xin isn't an avatar of the Hidden Moon, she's her own entity, albeit one that the Hidden Moon can speak and perform minor actions through, and even then the entire Moon Auntie's event was weird quasi dreamcape stuff)
Sublime Ancestors are an exception, by popping back into their physical form they can manifest their full power on the physical world they left behind
Also of note is that all the Sublime Ancestors of the Empire were bonded spirits, which probably isn't a coincidence
Spirits tend to have harder limits on how powerful they may naturally grow, and part of their benifits from being bonded to a cultivator is that it can help them to break through those limits and redefine themselves
Grandmother Serpent likely would not have ascended if she never met Yao
A better reference for a feat of Grandmother Serpent
after she became a Sublime Ancestor would be the one and only time she was awoken, when the Sage Emperor courted the White Serpent Queen he was forced to prove himself against Grandmother, whatever brief contest they had carved the bay of the Thousand Lakes
What are the 7 different sublime spirits? And what's the celestial pillar?
There's the Celestial Dragon the Grandmother Serpent, the Stone Ape, the Sleeping Isle, the Immortal Phoenix and the Red Garden are the onesI can think of.
There's also the Owl with prompted the Ministry of Integrity but we've been told that she/it/? hasn't ascended yet.
Current Sublime Ancestors of the Empire: The Celestial Dragon, Grandmother Serpent, The Reveler (Monkey), The Living Isle, and The Herald of Endings (Owl)
Previous Sublimes, now either dead or otherwise missing: The Purifying Sun (Phoenix), The Horned Lord (Elk), Grandfather of Tides (Crab)
There is also one prospective Sublime: Father Fortress (Scorpion)
And then there are the very rare Greater-Than-White entities that aren't Sublime Ancestors, like The God Tree Xiangmen, the Sunflower Goddess and possibly others we have yet to know of