It's an intriguing theory, but it relies on cherrypicking.

Looking at codex material, the Custodes outside the throne room wear armour as they are responsible for palace security. There is however an inner circle dubbed "Companions" (Codex Imperialis) or simply "the Three Hundred" (6E core rulebook), consisting of 300 elite Custodes (permanently assigned, not rotating), and it is these who function as the bare-chested honour guard for the Emperor's dessicated hull.

Said rulebook, by the way, also notes that the Warp rift is "kept closed" by the Emperor, although I generally find the notion of 300 bare-chested elite troops spending 10,000 years fighting a secret war with millions of horrible daemons from the Warp right beneath the Imperial Palace in general slightly exaggerated .

It all comes down to just how epic or realistic we want to see this setting, however, which is why the theory on 1d4chan is still an entirely valid idea. I may not appreciate this sort of "power level" in the heroes, but even I have to admit that there is something cool about such a war fought in complete secrecy.

The "epic vs realistic" notion also applies to this thread in general, of course. It can be summed up as having to pick between the heroic battles of the Horus Heresy novels, and the GW main studio's somewhat "tamer" representation. Just like one of them tells us that Primarchs can supposedly stand up to Titan-grade weapons, whilst in the other they are put n mortal danger by a random trooper with a plasma gun (then again, even the Emperor was almost killed by an ork).

Slight necro,and with respect to all of you, I think you're thinking about this wrong. You're trying to balance this against astartes and throwing certain descriptions to the wind (the time when what's his face watching the word bearers fought dozens of warp enhanced word bearers and only barely being brought down by argel tal comes to mind)

This is how I'd stat them:

WS95, BS80, S75, T75, AG70, INT70, PER85, WP90, FEL50

Traits: Unnatural Strength X3, Unnatural ToughnesX3, etc (don't have rulebooks on hand)

Artificer armor with +3 AP, iron halo with +10 protection, 5 histories chosen at will, Uniquely Crafted* Power Spear with the Fast, Power Field, Balanced, and either Shocking or Concussive (I seem to recall accounts of them knocking astartes around or stunning them), Uniquely Crafted* Bolter with Targeter and any specialist ammo, clip and weight halved. No gear comes to mind, maybe a +15 to autosenses and a +20 to strength not affected by unnatural strength, the black carapace trait for their armor, and perhaps various relic items of choice with the Uniquely Crafted quality

*: +15 to hit, +4 damage and pen, +1 Special Quality that makes sense for the weapon (no overheats for melee, no balanced for bolter)

All the functionality of a space marine's enhancements.

One other thing comes to mind: they should get a talent similar to Vindicare Assassin training for the dodges, but applied to parry instead.

Reason for all of this is that if we used the stats everyone else suggested, they'd be easily overpowered by things like bloodthirsters, a daemon they certainly fight every second under the golden throne. These guys are literally gods of war, in every sense, Valdor even outsparred *Horus*!

throwing certain descriptions to the wind

To be fair, this is often mandatory in a franchise beset by contradicting accounts and descriptions: you just have to pick the one you prefer, and live with the consequences of that choice. Just like with the Space Marines, the depiction of Custodes depends heavily on which sources you're looking at (especially once we're comparing, say, the Index Astartes with the Horus Heresy novels, which follow a power level that is worlds apart from the studio source).

That being said, I would generally try to avoid the "power creep" that comes from balancing units based on a "must always be able to beat X" philosophy, anyways, and rather examine what feels suitable based on their own description (depending on your preferred source).

I like Black Library's take on things most (though it should be noted that them and Lexicanum is all I really have access to, other than SM7th and IG7th.)

That being said, for a less brutal campaign (I prefer to run things where the killteam is always outgunned, outnumbered, and in some cases outsmarted) I'd go with one of the earlier statlines, those would cohese better, as a custodes slaying a daemon prince for instance, in one round, while lore-friendly, would far outshine the killteam's efforts over 4-5 rounds