Problem with LOCAL variables

[qWord]

And I failed to consider that the high loop count would "blend out" cache misses, when the only thing I can see that could make locals faster is a reduction in cache misses.

After the first round everything is cached for all variants. From this point on we are measuring the plain execution time of instructions and not the memory access - Isn't this what we want to measure when proving the postulation "it can also be assumed that locals are always cached"?
Looking on my result with your (unmodified) test, we can see that at least on my i7 there no difference between the global and local variant (the same as for your P4) - that's why I think we should make one-shoot measurements.