I got the MC5s today, with about 5 hours on them, and I'm liking them pretty nicely. Doesn't feel like these are the kind of headphones to benefit too much from burn-in, but I'll see. They are neither as harsh/aggressive in the highs or lacking in bass as I expected them to be. I agree with your impressions for the most part, with some differences in preference of course. Only thing I'm hearing differently so far is that timbre seems notably more accurate on the HJE900 than the MC5 right now. You have much better sources than I do though =P
I agree a lot that the temperature of the MC5 seems to shift between cool and warm. It must be the usage of a dynamic driver with etymoic tuning
Sometimes the impact on these surprised me, and they certainly extend down nicely. I can always hear the bass and sub-bass, it's just not as prominent down there as on the HJE900(obviously). I actually wish the treble were a bit more forward at the top, it seems to have a bit less sparkle-factor than my Pannies, though perhaps seeming a bit more controlled. They are also harder to drive than I expected. Obviously their ridiculously good isolation counteracts this, but my Pannies are a good deal louder from my cellphone than the MC5s are. Still, I have a rather weak portable source and their isolation lets me listen comfortably at any volume.
Soundstage and imaging are pretty much exactly as you described, I find. Only that I prefer the HJE900s larger, spatial presentation which others and I have described as quasi-holographic. In a way instruments feel relatively congested to me on the etys because of their smaller stage(which seems like it can extend nicely at the extremes, but lives around a much closer area). Perhaps because of a combination of the slightly slower decay and smaller soundstage, instruments seem less well separated; I can't quite position things as accurately.
These are my first treble-centric pieces of audio-gear though, and I'm quite enjoying them. The closest I've had in terms of signature are probably the PX200-II, which sound much like a more laid back, not quite a crisp version of these, slightly more on the warmer side. I don't think they do any aspect of sound or genre significantly better than my HJE900s so far, but I'm happy with my purchase to plan on keeping them at this point.