I'm a long time believer of burn in. When I first bought my IE8, I was able to listen to them contentedly for the first 50 hours, then noticed that they were getting increasingly sharper on the treble and duller on the lows. So in the end I decided enough was enough and just had them plugged into my PC and had them burnt in for a week. I did quick listening sessions everyday to see how they were coming along. And I noticed that the sound didn't change much at first, but after 300 hours they were starting to sound more and more like they should. I also have a similar experience with my AKG Q701. For the first 10 hours of listening, I hated them. I was really disappointed in the sound and even considered returning them. I thought I should just give them another try at 50 hours and I hated them even more. The music was sibilant and I almost gave up but remembering my IE8s, I kept going. At around 200 hours the top rounded off more and they were finally starting to sound better. Now I can't imagine ever letting them go. I will never sell them since not only are they the last of the Austrian versions, but worth more to me (sentimental value) than they are to anyone else. I'd get about 150 for them now. So I'd never sell them.
Some earphones I've experienced a lot less impact from burn in, with others none at all. I remember buying a pair of Shure SE535 long time ago as an upgrade to my SE530. Boy did I regret that decision. But in the end its what led me to my Q701, so I have no long term regrets. Long story short, they had piercing highs and I did whatever I could to try and tame them to no luck. Tried burning them in for 100 hours, and realizing that BAs don't burn in and noticing no difference in the sound whatsoever, I gave up and returned them.
Not all speakers and drivers are the same, and there are differences in their manufacturing tolerances and etc. So I suspect that that is why they sound different (of course design and materials factor here) and burn in differently. For me, I'd continue to believe in the process because I still have my IE8 and Q701 to this day and will continue to keep them long after the IE80 is discontinued and the successor is released. All because they sound right to me now, when they didn't before.
but your findings could most of the time go both ways, you just decide to interpret them thinking that it's not you, it's the IEM. but you have no evidence of that.
I'll take the dumbest example I can think of, when I first got the IE80 I often had my ears hurting after an hour or 2. but after about 3 weeks it didn't hurt anymore and I could use them for a longer period of time. that must be burn in.
see what I mean? there is what we feel, and there are facts, we do our best to put 2 and 2 together, but we tend to go with the rational that's convenient to us, not much for the solution that has the most probability to being right.
my reasons not to cry out burn in too fast when I notice a change in time:
1: we human suck bad at remembering anything accurately 3 month later unless it was a traumatizing event(anything exceptional helps), and even then the memory will only be more vivid, not automatically closer to reality.
2: before attributing changes to something not clearly proved or demonstrated for the IE80, how about going for reasons that are known? with new IEMs we don't yet know how to place them, we have to try a lot of tips, or sometimes just stick for a long time with the wrong tips, we try different depth insertions to find comfort before sound... all those choices do change sound and not just a little.
here are a few different tips and different insertions.I feel that's a compellng case for audible changes.
to be fair there is nothing proving it's really like that in your own ear or in mine, as the shape and length of the ear canal plays a great part in deciding the resonance points. but one thing is sure, there is a resonance point and it can be affected by what and how we put it in our ears.
I for one got used to care for that when using my first pair of ER4 many years ago. not going deep enough was creating some horrible harsh spike for me in the 7-8khz area. but just a little deeper, and all was fine. I almost sold it 50times before I found out it was depth that was making me like or dislike the sound. at some point I indeed thought the sound was becoming nicer with time.
the sibilance of the IE80 IMO is the same kind of problem, we all start with the crap default tips an it's hard to get a proper fit. plus the bass is so strong that we often feel like we don't need a proper seal to listen to this IEM. all those stuff take more or less time for us to find the best sound for us. I'm certainly inclined to believe those stuff to be responsible, or my bad memory? only when both can be cleared out(good luck with that) as non relevant will I start wondering about burn in.
But mine is generally set to the lowest. That bass can get crazy! It's amazing how clear they sound even with knob turned up to "post-apocalyptic nuclear mega-death". 
, except my right ear is different to left one & needs time to fit properly, Plus maybe as said in the previous posts Isolation / Fit / Seal could be 80% (I'm sure that fit is good, since the bass punch is excellent maintaing the sound), using small silicone tips which I found better than the M ones- bigger for my ears (will try the foamies later), didn't yet tried these outside for now, will do tomorrow and then compare the VSonics signature against these..