I am not sure where exactly I stand in this debate, mostly because, as you note, there is such a lack of definition.
But your emailer who writes that “The US [should] not use any techniques on terrorists that we do not use in the training of our own troops” doesn’t make a particularly strong suggestion in my view, because he ignores the significance of the context in which “torture” occurs.
Very obviously, it makes a huge difference in how one would experience a “procedure” depending upon whether it is done to you by (i) your own troops in a training setting in which you are sure you are going to survive or (ii) your sworn enemy in a hostile setting in which it is not clear you are going to survive. So the fact that some people will endure a procedure “voluntarily” in setting (i) doesn’t mean it doesn’t rise to the level of “torture” in setting (ii).
Consider this: does the fact that lots of people voluntarily undergo painful procedures in hospitals mean that we could do the same thing to detainees without “torturing” them? How about a relatively moderate example like a spinal tap? Would that be torture? Just imagine how scary that big needle would be in a detention room, while they threatened to insert it in your spine. I think this clearly demonstrates the proposition that context is crucial in the torture debate, not simply a description of the procedure in question. I imagine that McCain would endorse this point quite strongly.