+1
a huge way! People don't realize what physics does to high frequencies in a very small volume cavity. It's as wonky as bass is in an untreated room. I've always mused that the bass response from headphones coupled with the treble response of near field monitors would be one of the most linear and enjoyable, timbre correct experience there is. Customs usually result in a lesser volume of air between the spout and the eardrum - which shifts the primary resonance higher up. A resonance at 10-11k is much more tolerable than one in the sibilance region (IE 7K) for me at least. Similarly, customs are placed closer to the eardrum which helps negate the standing waves (jagged treble with peaks and nulls) where treble is most noticeable (lower and middle treble).
Keep in mind the above varies greatly with ear anatomy. While primary resonance can't be avoided, the standing waves effect can be ameliorated by the 'severity' of the second bend of the canal. Some folks can tolerate UIEMs better than others. Like you, David, I have long and straight canals with very little bend. This makes UIEMs VERY different from customs.
for me, comfort and sound quality is twice improved over universals. The only risk is in resale. But for me, it's worth it.