I have never heard any improvement in sound. I much prefer a nice soft dome tweeter. I may be old but those tweets that go way up in the frequency range make me edgy and nervous. I think it is the distortion or break up at " inaudible frequencies " really can be perceived.
I've heard a few soft domes that sound bright and edgy (for example, some from Polk) and metal domes that are smoother than a typical silk dome. Some high-end soft domes break up under 20 kHz. Why do they not sound edgy? Because it's all in the quality of the parts and implementation. I think many mistake upper midrange distortion as tweeter breakup.