Experience with adding super tweeter?

@kraftwerkturbo I'm afraid that is lay intuition kraftwork. If I run that test blinded very few people over 50 would be able to tell the difference. 

Good tweeters have no problem getting to 20 kHz. Many of them go higher. Of all the super systems I have heard, maybe 30 of them, not a single one had a super tweeter. Atma Sphere MA 2s have a wonderful reputation driving Sound Labs speakers. It is laughable because due to the amp's high output impedance and the super low impedance of ESLs at high frequencies the Sound Labs roll off at 6 dB/octave at 12 kHz and the people with this system never noticed. They think it is a great combo. It is below 12 kHz. 

"If I run that test blinded very few people over 50 would be able to tell the difference." I doubt that very much. If frequencies above say 12 kHz are missing, my money is on making a clear difference and high blind test colleration for 50+ audiophiles. 

With respect to speaker capabilities: a 12" driver can also 'get to 20 kHz'. The only question is HOW. There is a reason why some of your cited 'super systems' use beryllium for the material of their tweeter. Why not cast iron?

I am NOT claiming at all that super tweeters are needed. Instead, I was wondering WHAT they do when used either as 'add on' in addition to existing tweeter, or for example in a dedicated additional pass (band pass for existing tweeter, high pass for the super tweeter). Someting to consider for example to 'make up' for overdamped rooms?

I purchased the Aperion AMT version with their 6ft silver cables. At first, I was not sure if I liked them. Then, as I moved them and adjusted to where like a sub I wasn't "hearing" the tweeter, I found that I really liked them and did not return. Now that they are placed and blended well( -4db), I'll turn them off occasionally for confirmation and remain happy I kept them.

I may someday try the Enigma Sopranino here.  Very curious to see/hear if any improvements.