Realistically, you're right. There should be something. However, you're not going to be using the Sagaplate without orthstirr investment, because why would you?
If you were to use the Sagaplate without the orthstirr, then it would have completely different stats
In practice, true, but the 1/10 Orthstirr stats are actively bad. Like...our Gambeson is on par kind of bad. We might use it at low Orthstirr tithes if it had baseline stats and then added on top of them for Orthstirr but as things stand we never will. The entire bottom section of Orthstirr tithes is sort of cut off and thus wasted design space (and that's on top of the realism issue, of course). The defense numbers are also just kind of shitty in general. Like, it arguably makes up for it elsewhere, I guess but that number is way lower than even decent other armor, which seems wrong.
To be clear, I'm not suggesting a power up per se (well, except in Defense), just some sort of base stats and a lower increase per 1/10 Orthstirr (like, 15 Armor base +6 per 1/10 Orthstirr, and a base +3 Defense, or something like that).
Like, right now the 1/10 Orthstirr version is paying Odin for something arguably just worse than Good Mail + Mail Necked Helm (which is Armor 8 and +4 Defense)...which seems really off. If you're tithing any Orthstirr at all the armor should be actually impressive and right now it's just...super not at low levels.
As another example: the 30% level we have right now is worse than our normal armor IMO. That's partially because our normal armor is really good, but anyone who can make Sagaplate can make really good normal armor, so that still feels super off.
Overall Odin isn't a great God for the Sagaplate for Halla specifically, but it's a good example of what it can be. I'm guessing Thor would have something like Stamina and Attack bonuses which is probably better.
Logically, Thor would be Damage, and probably Resist. Maybe Stamina? All armor is gonna have Defense, so it's the other three that will vary.