Yes, but you forgot even for that, it's still small chance for her to do it.
https://typemoon.fandom.com/wiki/Amaterasu#cite_note-3
This is because in the information world legend becomes reality, and natural phenomena that have been incarnated as "Gods" can often hold more power than their original natural existence does.
Well even with that being said, if Berserker regained her sanity she would easily be considered a cheat-tier Servant. Her power, "The stage is the Moon, so all targets have their power reduced to a sixth of their usual." would be extremely useful against other similar cheat-tier Servants.
It's a conceptual numerical alteration, so it is unavoidable when on the Moon. Even the Moon Cell transformed version of BB would be limited by it.
In EXTRA and CCC, she is about the only one who is "Capable of defeating Konjiki Hakumen (Golden White Face), no matter how small the chance might be."
If we are talking about Pre Tsukihime Arc, which would be 100% power Arc, then pitting her against Servant Altjuna is useless as it is a stomp towards Altjuna unless he is at full power too.
Authority, on the other hand, would be very nerfed if she is a Servant. Many divine Servants suffer this too, like Ishtar for example. Ereshkigal can only use her Authority when she deploys her Noble Phantasm which literally turning the area into her underworld, and Melt can use hers as far as how powerful is (how much level she has).
Taking also the fact that Arc will really suffer if she is trapped in a sealed, separated space from Earth and Moon, like a Reality Marble for example. Someone like Emiya can even pick out on Arc if he uses UBW, as Arc is cut off from the Moon and Earth. Even Void Shiki too will suffer the same problem. In fact, she maybe as well powerless in such state.
Lastly, remember not every Divine Spirits come from the Earth. Amaterasu comes from the universe itself,as well as other sun gods like Surya or Apollo. Though this is just my speculations, i would say gods at the level of Shiva, Saver, Mara are the same too. And Arc's authority iirc is just her would get stronger at most one level of her opponent and Marble Phantasm. Even with those, she would suffer against someone like Gilgamesh that has many weapons
https://typemoon.fandom.com/wiki/Arcueid_Brunestud#cite_note-18
In the definition of Arcueid's strength, there's this thing where "her output changes according to her opponent". As an absolute order taken from her backup, the planet, she is allowed only an output a little stronger than her opponent. And. Single entity abilities of Arc and Servants are roughly on the same level
Servants use their respective Noble Phantasms while Arc uses her unlimited backup to fight, and differences occur depending on affinity. A simple-is-best Arc is an all rounder, and so generally her chances of winning are only high, but there are those opponents that she just has helplessly awful compatibility with.
For example, in cases where even if the guy's stats are about the same as Arc, he has a ridiculously large number of weapons with high versatility. As the amount Arc is allowed to take out is based on "single entity ability", against types like Gil-sama, well, you see?
And the "5 Servants + a" line is a comparison of simple "firepower". Like with the "corpses", foot soldier level opponents aren't going to be avoiding their attacks, so the ones who'd have the advantage are the ones who have more weapons. The reason why Servants excellent at one-on-one combat didn't stand out in Broad Bridge was because of this. Also, for normal Arc, she would get approximately the equivalent of 2 Servants single entity stats.
I'm just saying my statements here. I just say what i know about the two and the infos i already know