Since I had them both on hand for a few days, I did a direct comparison between my newly-acquired 007 Mk I's and my (just sold) Mk 2.5's.
I like the Mk I's better. There are differences in the sound but you can tell these are both variations on the same design.
The Mk 2.5 has quite different sounding bass. There's more midbass, and perhaps somewhat less very low bass (20 Hz bass) - I would say that this gives a somewhat "warm" sounding bass to the Mk 2.5 In some cases with classical music I found this added warmth was a good thing. For most of the music that I listen to, however, I did not care to this approach for bass sound. I didn't like it, but I think some people might.
There is also something different about the treble on the Mk 2.5 . It doesn't sound as smooth as the Mk I. Perhaps the Mk 2.5 is a little brighter? But I'm not sure I'd really say brighter. But there is a difference. Maybe it's a time-domain thing and not a frequency response thing? A little more low-level high frequency ringing with the Mk 2.5? I can't quite pin it down. But the Mk I sounds more neutral overall to my ears. It's almost as if the Mk I was voiced to be neutral, and the Mk 2.5 was voiced to be a little warm and then they did something to the treble to compliment the warmth.
I wouldn't say, just flat out, that the Mk I is a better headphone than the Mk 2.5 - I will say that I LIKE the Mk I better. If possible, buyers should listen to both before making a purchase. You might like the Mk I or maybe you'll like the Mk 2 or the Mk 2.5, only way to tell is to listen.
Thing is, that is really hard to do.
Maybe at the next Can-Jam a listening station with all three can be set up. It's an idea, anyway.