[...] Perhaps I've a placebo effect bias, but I've the feeling that the IR are better "wrapping" the sound, and the mix of direct sound and reflections sounds more "natural" to me.
Thanks for the friendly feedback! Better enveloping and and more "natural" space is indeed the intended effect of the IEM*) upscaling process. The effect is maybe less obvious in stereo, but it's spectacular in surround and especially 3D.
*) As a matter of fact there's even quite some academic literature available on this topic, most notably by the team around Prof. Zotter, who is one of the driving forces behind the Ambisonics research "hotspot" at IEM (Institute of Electronic Music and Acoustics in Graz).
/Dietz - Vienna Symphonic Library