Surely a stamped metal screen is going to resonate more than moulded plastic. I can imagine the sound each would make if you strung them up on a thread and hit them with a donger. [If you try this, best be sure no friends & family are nearby or you may confirm their darkest fears about the impact of this headphone obsession on your mental health
]. Carbon fibre is not immune to resonance either. The reason it's used in tennis rackets and racing cars is because it is harder and doesn't flex as much when stressed. As a consequence it resonates at higher frequencies. The important point would be whether its damping factor is better than the standard plastic. Personally I don't care because I think the whole thing about resonances is just a sales pitch. Stylistically, I like the pattern of the HD580's covers which could be from a brick wall in a vineyard somewhere in the mediterranean. From the practical viewpoint, I've seen a dented metal screen on a demo HD600.
John_jcb's point about driver selection is more of a concern. For example, if you buy a 5% tolerance resistor you can expect that it will be at least 1% wrong, otherwise it would have had different stripes painted on it and been put in the 1% basket. But there's no way HD580's are mismatched. Obviously all the drivers are being measured and those for the HD600 may indeed come from the middle of the distribution, but there's no reason you can't pair off the remainder. If this were not the case the difference would be audible, both in the headphones and here on Head-Fi. Provided you discard the drivers which don't meet the specification all will be well.
As far as the sound, I have heard subtle differences in a demo environment, where an HD600 had a slightly bigger bass hump and slightly more of the signature "airiness" to its top end than an adjacent HD580, giving me a preference for the latter. Such a test says more about the particular phones I listened to than the models in general. The primary problem is to get the volume levels identical, which, if there is a genuine difference in frequency response between the phones being compared, will depend on the source material. Then something as trivial as the adjustment of the headband can affect the perception of soundstage, etc. etc.
This is an interesting thread and I hope someone at Sennheiser is reading... It's all very well to change the trim and produce a deluxe version for those with the appetite, but to try and milk the market by creating an artificial shortage of the basic model would be a short-term strategy which could easily backfire. One day Senn's competitors may get a grip on diffuse field equalisation and laser interferometry and...
Hey what? Just caught myself in a pipe dream. Sorry. Feeling better now. HD580/600 rules eternal!