beryllium vs diamond

Today's dome tweeters are way superior to what we had 20 years ago primarily because their power handling has improved to the degree that you can get to 100+ dB without blowing them. For a point source system a single tweeter is optimal. Arrays with dome tweeters just do not work well primarily because you can not get good ones close enough to form a smooth linear array. To do this at 20 KHz the edge of the domes has to be closer than 15 mm. Not easy which is why ribbons make far better linear arrays. Will they ever be as good as an electrostatic speaker? If you prefer point source speakers sure. If not then never. ESLs are better than ribbons because their acoustic impedance more closely matches that of air and you can ditch the crossovers. The downside is that you wind up with a very large loudspeaker. 

Will they ever be as good as an electrostatic speaker?

And ESL's will never be as good as Plasma tweeters. I have a pair of Plasma MP-02's crossed over from my ESL panels.
You've never heard/seen highs "dance" in front of like fireflies you till you've experienced Plasma tweeters.
It's all about the "diaphragm mass" when it comes to producing the upper mids/highs, and a Plasma flame has no mass.
https://www.ebay.com/itm/Magnat-Plasma-MP-02-ion-tweeter-massless-air-plasma-speaker-Plasmahochtoner/272894087984?hash=item3f89c15330:g:V5sAAOSwCJxZ6PXy

Cheers George

They are interesting to say the least. I was in on the ground floor with Hill Plasmatronics helium tanks and all. I am not sure what those are but they are not plasma speakers. To create a plasma you need some serious heat which requires serious power and an inert gas. Those things would melt. Aside from that they are not line sources so they will not radiate in a way to match a line source. Maybe they will work well with a point source but you would have to get them close to the midrange driver which would make them impractical to use with most point source speakers. 

I am not sure what those are but they are not plasma speakers. To create a plasma you need some serious heat which requires serious power and an inert gas. Those things would melt.

You need to do some homework, they are Plasma, and one of the best.
There are pages on this German Plasma tweeter if you take the time to look.
https://img.aussieaudiomart.com/uploads/large/823697-magnat-mp02-plasma-tweeters.jpg
http://www.plasmatweeter.de/magnat.htm

Oh, and georgehifi it is the mass of air moved per unit weight. Since ESLs have such a large surface area the mass of air moved is quite high even when compared to ribbon tweeters and the mylar diaphragm weights much less than a comparably sized sheet of aluminum. The issue with ESLs has always been beaming. Unless you sit right in front of the diaphragm you hear no high end at all. ML deals with this by curving the diaphragm which greatly limits the low frequency capability of the panel. Sanders just makes a wider panel and says to sit right in line with the speakers and you shouldn't be listening any where else anyway. I am not kidding. Check out his web site. Acoustat made narrower panels and angled them. My 2+2s are two panels wide and only disperse 20 degrees at best so in reality you have to sit in line with them. They did make 3+3s and 4+4s which were really huge. Soundlabs uses a facet system that covers 45 degrees so you will have even treble over the size of a 3 person sofa. This is probably the best solution but it still results in a very large speaker. There have been attempts to electrically decrease the size or width of the tweeter area like Quad and Acoustat eventually played with but this causes other problems which I think interfere with the magical quality ESLs can have. No speaker can do the 1 way thing as well as a large ESL. Cross overs are a pain.