The problem with determining the effect of supersonics, is that all of these supertweeter units have significant output into the audible treble range. And it’s hard to discern whether we’re hearing the effect of that, or the actual supersonic frequencies. The Townshend looks like it attempts to shelve down treble below 20K, which is a good idea to mitigate comb filtering and "peakiness". But those levels could still be audible.
The Tannoy units cross over high (16kHz - 20kHz) in an attempt to avoid the issues of audible treble, but it’s not enough. It’s VERY audible. At the recommended settings for my speakers (I’ve had 3 pairs of tannoys and 3 supertweeters tried in combination), it sounded like a little treble blaster. Way, way too hot for most sources. It did however, go well with old needledrop tapes - these benefitted from the treble boost! In general, I always ended up reducing level significantly below the recommended settings (e.g. 91dB ST level for 95dB speakers). In the end, I stopped using them, every time. The "air" effect on metal percussion was nice, and the sweet spot was expanded - but in Tannoy’s case, it also came at the expense of coherence and increased listening fatigue. However, I see how it could be the right solution for some systems.