Not sure where you’ve been reading, but the effect that torsional stiffness can have on ball flight is minimal. The effect on feel is a bigger factor. The only way you might see an effect on ball flight is if you were to have a very high club speed player hit a shaft that had a very high torque, like 12 degrees or higher. Fortunately there aren’t any shafts on the market with that kind of torque. If that same player did hit say a 6 degree torque shaft, of which there are a few, they would feel softer (too soft most likely). In our academy we have a shaft with 14 degrees of torque and one with less than 2 degrees. We have students hit and compare and most will definitely see and feel the difference. When comparing a 3.5 degree to a 5 degree, most students can not tell the difference and there is no visual variation in ball flight. Important to understand that the torsional stiffness of a shaft (the torque) is measured in the raw shaft at the raw length. Once a shaft is cut from the tip (if required) and installed into a hosel, the actual torque will change. As a very general rule, for every 1/2″ the shaft is installed into a hosel, the torsional stiffness will reduce by .2 degrees of torque. Also, torque is measured by differing methods from manufacturer to manufacturer. There is not standard measuring method. That being said, the differences in the methods don’t amount to much difference in the readings.
We have always used the torsional stiffness number as a way to distinguish a slight variance in the feel of the shaft. For instance, if you have two S flex shafts and one has a 3.5 degree torque and the other has a 4.5, the 4.5 degree shaft might feel slightly more flexible or softer, assuming the shaft profiles and weights are similar otherwise. Very difficult to see, in our experience, measurable difference in ball flight that was caused by the 1 degree difference in the torque.
One other thing, in your research and the conclusion that lower torque shafts will help with hooks and higher torque shafts will help with slices, that might only be a very generally accurate statement IF the higher torque shafts were also more flexible in their overall profile (which most higher torque shafts are). In your numbers listed, 4.0, 3.5, 3.0, 2.9 etc., if you have good sense of feel, you might possibly notice a difference in the feel between the 2.9 or the 3.0 and the 4.0 (maybe). Doubtful you would notice any difference between the 3.5 and the 4.0. Again, this assumes there are not other factors in the shaft designs you are comparing that are contributing to any feel differences.