Samantha @ Clek has added a comment to the Facebook thread in the last hour which clarifies the testing. What I did learn from her comment was that, the height of a child absolutely can affect how the seat performs RF. It will rebound more with a taller child. Thus, when a manufacturer puts a RF height limit on a seat, I will follow it and NOT go by the 1" from top of shell rule.
That being said, I've always been one for following the RF height limits. In cases like the Complete Air where the Canadian RF limit is 36" I think it is, and the U.S. is 40", I'm guessing the seat doesn't pass our rebound standards at the U.S. limits. So I'd definitely not go higher, nor would I go to 1" to top of shell just because it's tall.
And yes there are different torso heights, and hopefully there is some leeway in torso height for those overall RF height limits.