Theoretically I suppose as the argon content in the gas goes up, the recovery of elements across the arc increases, predominantly Cr and Mo which would be those alloying elements most likely to be oxidized I think. This, it seems to me would increase the corrosion resistance, the tensile strength, and the ferrite, since Mo and Cr are ferrite stabilizers. But in all, I think the difference would be negligable. IMO, if the stuff welds well I'm not sure what the problem would actually be.
The biggest issue would be, as Tim mentioned, puddle edge wetting and edge penetration, since the mass of argon tends toward a more fingered narrow arc profile. CO2 broadens the penetration pattern. Or perhaps that those compounds in the flux intended for deoxidation would end up either in the slag or the weld metal.
Is this critical?
I would have to say that IMO the advantage of the heavy CO2 mixes for SS is economics more the metallurgy or mechanicals. The disadvantage of a higher argon mix would be the greater skill required to ensure good fusion under the entire bead from toe to toe.