Experience with adding super tweeter?

Goofy idea....unless you get into active crossover implementation or mod the existing passive crossover. 

(It is nothing like blending in a subwoofer without bass management and getting away with it).

The PHASE thing: at 10kHz, the wavelength is 3.4 cm. So moving the supertweeter through that entire range is not too difficult, maybe in 10 steps of 3mm or so. 

But as with any OTHER speaker, the PHASE will only be "correct" (in line with other speakers) for one specifc frequency. So I guess speaker manufacturers may try to get the drivers 'into phase' at the crossover frequency? Or simply just ignore the it since it is OFF the whole time except for one frequency?

The phase thing is best established by trial and error. When it locks in, that is actually quite easily audible

If im not mistaken,

I believe a plasma tweeter can achieve performance well beyond that of a super tweeter. They can achieve 150kHz range without the downsides.

I’ve been infatuated by plasma since I saw demo but the community will be restricted unfortunately as these speakers typically come with six figure price tags

no inductive voice coil, no inhomogeneous magnetic field, no resonance-bound membrane, no mass-spring system vibrating back and forth, and no non-linear suspension. All there is light and air.

also can be hazardous unless designed brilliantly 

I remember the plasma. Ingenious (no mass!). But widespread use I think was also hampered by the Ozone (O3) that plasma 'flame' generates. Small doses are healthy, but higher/longer can cause problems (cance) IIRC?

Ozone (O₃) is a colorless to blue gas with a pungent odor. Exposure to ozone may cause headaches, coughing, dry throat, shortness of breath, a heavy feeling in chest,  and fluid in the lungs. Higher levels of exposure can lead to more severe symptoms. Chronic exposure may lead to asthma. Workers may be harmed from exposure to ozone. The level of exposure depends upon the dose, duration, and work being done.

Ozone is used in many industries. It is used for purifying air and drinking water, in industrial waste treatment, oils, bleaching and waxes, and to make other chemicals. Some examples of workers at risk of being exposed to ozone include the following:

• Outdoor workers in areas with high levels of ozone
• Factory workers in paper and pulp mills
• Workers in waste water treatment plants
• Fisheries workers who treat storage water