With my Westone 3's, Triple.Fi's, and my former SE530's, I have noticed some slight changes over time and sometimes they would sound the way I remembered them to be with various listening sessions (using the same tips of course). This leads me to believe that it was just a mental phenomenon that was occurring when I perceived slight changes. Even with my Monster Turbines which uses a dynamic driver, I noticed some small changes, but I can beleive that some brain adjustments were going on.
However the Sennheiser IE8 and IE7 are anomalies. Out of the box they both didn't sound great at all. In fact, I was contemplating on returning them because they were definitely not worth the price that I paid for them. I decided to 'burn' them in for a couple of weeks and then make my final decision. Still using the same tips I noticed dramatic improvements. Bass tightened and treble opened up. It was in effect, a metamorphosis of the sound signature. I could believe that it was a mental thing if only some slight changes occurred but the opposite is true for the IE7's and IE8's. I'm not sure if 'burn-in' or 'break-in' is responsible but due to my own experiences I can't believe that my brain was adjusting to these iems unless there is some sort of audio illusionary effect that is going on in particular with the IE7's and IE8's. If that is the case, then Sennheiser is a master audio illusionist. The modern day Houdini's of the audiophile communities.
I personally value both scientific evidence and human experience. Without human experience, there would be no scientific method.