Flyfshr,

If thread manufacturers listed their thread by breaking strength and diameter (like fishing line manufacturers do) we would have some useful information that would allow us to actually compare different fly tying threads.
Measuring diameter of monofilament is easy. Measuring diameter of braided line a little trickier, but still very do-able.

Measuring the diameter of multi-strand twisted tying thread is dark magic, and of questionable use, unless first the number of twists per unit length is agreed across manufacturers.

Cheers,
Hans W