The KOS-MOS Archetype, the original version constructed, was fairly more powerful than the current test version (seen in Episode I). The archetype utterly slaughtered Vector personnel and heavily armed U-TIC soldiers, and took down a ship with no effort (minute 2:18 of video).
Video to search: Xenosaga I - 31 - Cherenkov and KOS-MOS
However, KOS-MOS can enter a blue eyed mode. In this mode, Mary overpowers KOS-MOS mock personality and takes control of the body, but she doesn't awaken as herself completely. The only time she entered this state in Episode I she pulled the feat of absorbing and thus destroying a massive fleet of Gnosis by herself.
Video of the feat: Xenosaga I - 46 - Escaping the Gnosis
Later in Episode I, KOS-MOS obtains a Phase Transfer Cannon, a weapon that is said to be able to vaporize a star (minute 5:50). Shion's statement (minute 3:50). It was concentrated in single point to not destroy everything arround KOS-MOS. This cannon puts KOS-MOS at Star Buster level or even System Buster if the cannon is used on the Sun.
Video-Xenosaga I - 61 - Revealing the Song
In a vision shown by Nephilim to the party, the completed version of KOS-MOS is seen armed with her Tertiary Weapon System (in a possible future) and using it to fight with U-DO, who is God. So KOS-MOS gave a hell of a fight to a portion of God's waves, and the collision of attacks seemed like busting a whole Galaxy and beyond. (All of this in minute 1:10 of the video).
Video- Xenosaga I - 56 - Febronia (Part 2)
In Episode II, we are shown that KOS-MOS can summon high tech rifles (minute 4:35) and restrain Albedo after he linked with U-DO using the Tertiary Weapon System (this is Combat Frame KOS-MOS, not a test frame like in Episode I) (Minute 6:20 of the video).
Videos in order of feats:
Xenosaga II - 33 - Cecile and Cathe
Xenosaga II - 36 - Martyr
KOS-MOS has access to a high tech motorbike (minute 2:08) which can shoot lasers and merge with E.S. Dinah. After this, KOS-MOS enters blue eyed mode and powers the E.S. so it can destroy the all the attacking auto-tech (from minute 3:18 all this feats).
Video-Xenosaga II - 26 - KOS-MOS's Rescue
In Episode III, KOS-MOS gets upgraded to the Version 3 of her frame and becomes much more powerful, but gets easily overpowered by Omega Res Novae (more on this one later) and later by T-elos, her "ultimate version" created from the true body of Mary.
Videos-Xenosaga III - 026 - Chapter 2 - KOS-MOS vs. Omega Res Novae
Xenosaga III - 051 - Chapter 3 - KOS-MOS vs. T-elos
The power of KOS-MOS Tertiary Weapon System is the only thing apart from Omega Res Novae Phase Transfer Cannon that can penetrate the Hypersphere that surrounds Rennes-le-Chateau (minute 0:35)
Video-Xenosaga III - 045 - Chapter 3 - Penetrating The Hypersphere
After being destroyed by T-elos, the frame of KOS-MOS was upgraded to final version, making her a rival to even a Testament.
Video-Xenosaga III - 074 - Chapter 6 - KOS-MOS vs Black Testament.
It's revealed that KOS-MOS power comes from the Zohar and thus from U-DO (God himself).
In this new version and after fully awakening as Mary, KOS-MOS is finally able to overpower T-elos. Now KOS-MOS seems to have a more compact version of her star busting Phase Transfer Cannon incorporated.
Videos-Xenosaga III - 139 - Chapter 9 - KOS-MOS Awakens
Xenosaga III - 140 - Chapter 9 - KOS-MOS versus T-elos
After this, KOS-MOS absorbs the Animus power from T-elos (Mary's body) and becomes a god-like being, with the power to shape Anima at will and to direct the consciousness of people. Once KOS-MOS obtained it, people arround all the universe became Gnosis and were attracted to were KOS-MOS was, to Zarathustra.
Mary Magdalene was a god being born from the Collective Unconsciouss of humanity along with Yeshua and Wilhelm. She was the real domain form of the power of Animus. After obtaining Animus, there is the posibility that KOS-MOS obtained virtual inmortality, the ability to bend the laws of physics and to affect everything from the imagininary domain like Yeshua and Wilhelm can do, but there is nothing that indicates universe busting power like that of Yeshua.
Mary is possible reincarnation of Eve, if not outright confirmed in Episode III.
So yeah, KOS-MOS is more than a cute and sexy android guys and I hate when people think she is just fanservice.
Note, this is not a KOS-MOS respect thread. I will add more characters as time passes.