Both me and Alleo have said there are degrees of being Tempered. However, there is not degrees between not being Tempered and being Tempered. Being Tempered a little is still being Tempered and it always effects your mind.
And so far, it has always been as simple as Tempered/Not Tempered. The Ascians have been Tempered longer then anyone else and they are extremely rational. They are also obviously under Zodiark's Thrall even though they have in-character motivations and desires to do what Zoidark wants them to do. Which is the same thing any other Primal wants their Tempered to do: keep them in the physical realm and supply them with aether.
Where we don't see a comparison between the Tempered and Sin Eaters is that there is a degrees of how close to being a Sin Eater a being is. And we don't see that with Tempering ever.