@timlub --
What it comes down to is... really good mids with any type of top end extension without cone break up is hard to find in true high sensitivity speakers.
Perceived really good mids and upper range extension (i.e.: "any type of top end extension" leaves room for quite a lot) is readily available with true high sensitivity speakers - without necessarily requiring exotic cone materials. Being so I gather cone break-up, in whatever shape it may present itself (or not), isn't an issue.
@unsound --
One will be hard pressed to find high sensitivity speakers that can produce respectable step response or square wave response.
Question is whether it holds significant correlation in regards to perceived sound quality, not least depending on who you're asking. There's hardly consensus here, but transient behavior in (time aligned) all-horn set-ups, representing the best efficiency, are hardly the ones to fault the most in this regard. Even so, relevance/correlation is required.
There's a lot more to speakers than sensitivity alone.
Sure, but I'll definitely side with poster @alexberger's above posts on the significance of high sensitivity (and headroom) as an aspect highly overlooked by the audiophile community at large, and that it's an essential ingredient in achieving a live sound imprinting. Those who've heard a well-implemented all-horn system covering most of the audible range will know this kind of sonic ignition, scale, presence and ease simply doesn't exist in low to moderately sensitive speakers; they just sound restrained and malnourished by comparison, not that they can't be highly capable in other areas. Yes, high efficiency and full-range requires BIG size, but you want to eat your cake too that's the pill to swallow. Personally I find it's definitely worth it.