Experience with adding super tweeter?

"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.