I'd say if IEMs tend to create greater pressure, you'll be able to feel it, won't you?
Now, an 85dB SPL sound wave played by a speaker, compared to that played by the IEM may differ once it enters the ear canal, but I think the ultimate factor is whats the pressure created on the eardrum.
That's my understanding. I have a decent amount of education in biology, but from what I understand, since "pressure" and "sound" are literally the exact same thing and the pressure from a wave is what you actually hear, increased pressure from IEMs is why they generally are louder and have punchier sonic profiles. I'm sure there is some credence to the idea of it reflecting and compounding inside the ear canal, but I can't imagine this would magically create an imperceptible increase in volume.
Apparently, they may. Especially IEMs. You guys should have a look at this publication (http://www.aes.org/e-lib/browse.cfm?elib=15786). Basically it states that when sound waves get trapped in sealed ear canal, air pressure builds up inside and can boost the signal even up to 50dB! This leads to ear drum excursions up to 1000 times greater than in normal listening conditions. That's pretty scary to say the least. To make things worse, this build up triggers Stapedius reflex, which makes you perceive the loudness much lower than it actually is. This publication is pretty new and I really hope that more proper studies will be made in the near future regarding IEM safety, because now it looks like we still know very little.
When you posted this, I did some personal research on the Stapedius reflex, and the sources I read almost universally stated that the Stapedius reflex, being mechanical, makes you perceive loudness as lower because you are literally just hearing less. It tightens your eardrum and pulls your stirrup and mallet away to make sound waves provoke less of a response in your hearing system, and if it theoretically stops you from hearing a quiet noise, that means the noise has no effect on any structure of your ear. (Møller, Aage (2000). Hearing: Its Physiology and Pathophysiology (illustrated ed.). Academic Press. pp. 181-90. ISBN 0125042558 is the source that was cited.) Apparently, the Stapedius reflex can lower the received intensity up to 20dB, and it occurs the second before you start to vocalize in anticipation of speech or sound to reduce the volume of your vocalization to you, as well as in situations where your brain hears a particularly loud noise. There were a few sources that showed that vocalizing before the onset of a damaging sound reduced the effect on the ear because the intensity of the sound was muffled by the effects of the reflex.
I have done and often read a lot of research in different scientific fields, and I can tell you that a single study, no matter how robust, is literally useless and meaningless to anyone with experience in the field until replicated. Without a lot of verification, they also tend to lead to things like the perception that aspartame causes cancer (proven false in any amount humans can physically consume without direct ingestion of the chemical itself) or that vaccines cause autism.
Just my wary 2 cents.