Fair point, but I for one would rather play a Great Crusade game that leads into the Heresy than just a Horus Heresy game, so enemy Primarchs wouldn't be applicable for half or more of the campaign.

But… we do have at least a couple very good markers for a Primarch's ability based on some things that appear in the wargame already:

Sanguinus vs a Bloodthirster - which was NOT an easy fight. Sanguinus got the better of it, but keep in mind that wasn't even Round 1 (We'll know more about Round 1 soon enough when the Blood Angles Heresy book lands later this month)

One of the Primarchs (I forget if it was Alpharius or Fulgrim or possibly still another one…) vs an Eldar Avatar - again, it was NOT an easy fight. Though again, the Primarch was better.

And I'm pretty sure some of the short stories have seen a Primarch against at least a Lord of Change and another against a Keeper of Secrets. (Or at least one of the above - the short stories blur a bit for me though)

In all cases, yes, the Primarch wins. But in general they're not easy fights by any stretch.

And of course we know Chaos Infused (Though already wounded by Sanguinus!) Horus is enough to nearly beat a reluctant Emperor.

So regardless, we do have some scale to work with there with Wargame Conversions in regards to fights against Top Tier enemy types in said wargame.

That said I doubt I'd be surprised if Primarch Stats work out to be somewhere in the realm of straight 10s. Possibly a few 8s or 9s here and there depending on the Primarch.

In the fights you reference… Ka'Bandha, the Bloodthirster who Sanguinius broke over his knee at the Siege of Terrra, previously broke (other words used are "snapped" and "crushed", depending on source) Sanguinius's legs and forced the Primarch and his Legion from the Daemon World of Signus Prime. It's also theorized that Ka'Bandha might be responsible for the Black Rage. As for Fulgrim versus the Avatar of Khaine… well, Fulgrim owned that particular Avatar. He killed it with his hands. Melted off his power armor's gauntlets and left his hands blackened and blistered, but he literally punched its face off. As for Horus versus the Big E, it's worth noting that Lupercal was infused with the power and influence of all four of the Ruinous Powers, and Papa E wasn't willing to accept that his son had truly fallen until Ollanius Pius (who was a Guardsman, dammit. Not an Imperial Fist Terminator, not a Custodes, but a normal, run of the mill Guardsman) stepped up to the plate, promptly got his face stomped with a LOOK from Horus, and made the Emperor realize that all hope was lost. Then, pew pew psyker beams, Horus be dead.

Hey, I did say against a reluctant Emperor! And again, remember Sanguinus made the dent/hole in the armor that helped win…

But back on the Bloodthirster, I'll also add this: Greater Daemons are another of those special snowflakes of the setting that never really gets handled that consistently. In some they're able to be taken down by massed Tankfire… in other situations one of them can pretty much melt entire regiments of Guardsmen with a look. Heck, even a greater Daemon's size seems… random. Or their comparisons to Daemon Princes (Sometimes greater than, other times far weaker then…)

Granted, it makes sense if there's tiers of Greater Daemondom too. But those aren't demarcated in the setting anywhere.

On a purely theoretical level, it's even kinda cool to think of the Primarchs as the Emperor's "Greater Daemons"