Let me open by saying: No offense/challenge intended:
Why does Spiral ring beat at least one Terrasoul? With such a large modifier on VIT wouldn't it be better to stack that where possible? Or are you calculating that the STR modifies overall attack numbers more favorably after a certain point?
2 VIT will be 1-2 base damage on Upheaval. 5 STR will be 1.25 fSTR when uncapped. The average difference ends up being about 0.5 base damage in Terrasoul's favor, but Spiral will also add 3-4 attack. D on Upheaval is going to be pretty high given weapon type and WSC%, so the attack will pull ahead slightly as long as you're not capping cRatio.
Note that this is looking at average cases. You can determine the specific gain from WSC and gear accordingly since it's not dependent on mob stats.