Do supertweeters matter when we cant hear past 15,000 hz?

Therefore....is the addition of a super tweeter a benefit to my speakers

Yes, it is.

I have more practical question: Coral h-104 vs Fostex T900a vs T500amk2 - which one are better as a pure tweeter for Avatgarde Due horns. Or may be something else is better?

"Hearing" concerns the conscious notion that we are aware of sound, and the unconscious response to sounds that we may not recognize as such.  Our hearing mechanism is mechanical- electrical.  Specifically sound activates the mechanical elements of our auditory system which convert mechanical movement to an electrical impulse sent to the brain.  Our brain recognizes impulses within a specific bandwidth as "sound", but does not consciously provide a response to impulses outside of the bandwidth range.  But- our auditory mechanism responds to sound outside of the normal range of hearing, and our brains still receive an electrical impulse from the out of range stimulus.  While we are not conscious of a "sound" generated by out of bandwidth signal, we do acknowledge a response at the subconscious level.

3 landmark studies proved that we can respond to content above 20khz.  In the early 1950s the US Army tested the hearing of soldiers and determined that while we can not "hear" content above 20khz, we are aware of it when it is removed from program material.  Soldiers reported that something had changed, or was missing from the content, but they could not fully describe what was missing.  Just that is was "something".  This study and findings were reaffirmed by CalTech in the 90's, and by Swedish researchers about 10yrs ago.

Just about everyone who has reviewed a super tweeter reported that even though they could not "hear" the super tweeter, nor could the pinpoint specific sounds as coming from the super tweeter, felt that it added something beneficial to the overall content.  And they definitely could tell when the super tweeter was silent or removed.

Whether or not content above 20khz is important to you, it can be a factor in accurate reproduction of recorded music.  Brass and string instruments produce overtones that extend beyond 20khz and can reach up to 40khz.  16/44 digital introduces a brick wall filter at 22khz that abruptly cuts off the harmonics, and causes phase anomalies and odd order distortion to be reflected back into the audible range.  How ?  Picture a pool of still water, the surface looks like glass.  Then drop an object into the center.  Ripples will emanate in concentric circles from the point of impact, and flow outward until they come into contact with the wall of the pool.  Upon impact with the wall, the ripples are reflected back towards the center, but are out of phase with the ripples still coming from the center.  Some of the ripples will cancel each other, some will enhance each other, but the overall effect will be very noticeable.

HiRez digital allows for the filter to be moved out to the point where it no longer interferes with extended instrument harmonics.  24/96 extends it to 48khz, and higher sampling rates move the wall further outside the audible range.   Listeners almost universally report that hirez sounds smoother, more natural, and more analogue like compared to 16/44 Red Book.

A long winded answer, but I hope it helps.

I’d wager any audibly perceived difference for the better with the addition of a "super tweeter" has little to none to do with frequencies extending 17-20kHz, but rather that the tweeter in question is better at handling the frequencies that are actually audible in the upper octaves. Equally important (even more so, I’d say), is the reproduction of the lower spectrum of the tweeter’s working area, and how this meets the with the midrange. To my ears most, if not all direct radiating dome tweeter lack energy so to, perhaps paradoxically, make them sound less like tweeters, and more like something that’s a natural extension of the midrange. Mention of frequencies that extend way beyond 20kHz, together with their claimed importance, seem more like marketing ploy than anything of real significance.