Super Tweeters

I have Tannoy ST100 on top of Esotar tweeter in modified Merlin VSM. Highly recommended. I suggest the version with the lower 14, 16, and 18kHz crossover points. This allows the roll-off to extend into the more audible mid-treble range if need be.

The Townsend ribbon Super tweeters that Ozzy mentions are widely praised. I know that I want a pair.

you may also look " home" for some JBL 2405 and with 2uF cap in series you can hear what it is all about.....

Super tweeters need to be design for particular loudspeaker used not just off the shelf one size fits all. More to it than just setting a box on your loudspeakers and saying good enough if you understand how they interact with the rest of the loudspeaker you would know I speak the truth. But posting such isn't what you all want to hear so it will be ignored. Forum posters want someone to agree that what there doing is right and generally only consider posts that agree to be of use. Sometimes I wonder why I bother so happy listening slap that ST on top its all good.

Johnk,
To begin with, a lot of these supertweeters do have adjustments on them, and since you are only augmenting the highest frequencies that are likely not even being produced by your speakers tweeter, it is not like you were adding a "super midrange" (if such a thing existed) which would cause all sorts of problems obviously.
In the end, many have found them beneficial to the whole sonic spectrum, and if that conflicts with measurements that say it shouldn't be so, who really cares, except apparently yourself.