Have a look at the IMR thread (another British IEM brand with a cult following; mostly multi-driver hybrid models, and an 'interesting' approach to release scheduling and communication). Over there they talk about waiting >500 hours until the IEMs finally come good, and a right rollercoaster ride to get there.I didn't believe in burn-in until I got the Atrios, which started very muddy then got much better. Of course, it could also be just in my head -- maybe I simply got used to the sound signature -- but then why did I experience this only with this IEM? So while I'm still not 100% sure about burn in, I still lean toward it being real, and quite important for some IEMs with a dynamic driver.
In fact, if you pull up a frequency response curve and replace frequency with time on the x-axis, that's basically the emotional journey IMR fans go on with each new model!
So, my belief system is capable of accepting the phenomenon that modern dynamic drivers need play time to become sufficiently flexible before they achieve the performance that the designer had in mind.
I think there's some of this going on with my Flare R model - they seem to be clearing up, especially in the treble region. YMMV though of course.