"C. J. Clegg" wrote

Which is the best amateur-band automatic antenna tuner made, for use
with a 160-meter inverted V with open wire feeders, to cover 160
through 6 meters?

Most ATU's could tune your rain gutter with about equally bad performance
that you would get on any single antenna that you asked to service that
entire bandwidth.

Also, is there much perceptible difference in performance, loss,
signal strength, etc. between the best of the best and the ones that
are only average in quality, assuming that both will properly tune the
antenna?

I have never read reviews that compared them against each other.
http://www.eham.net/reviews/ will display the comments you're looking for.
After eight months of use, I am completely pleased with my MFJ-994 600w ATU.

Jack Painter
Virginia Beach, Virginia