I'd be quite suprised if O2 sounded even a bit better with OPA627. That part would actually be inferior for IEM use due to higher current noise, similar voltage noise. It'd be "meh" for headphones.
Changing the capacitor could've reduced ESR pushing the compensation a bit up in frequency, however this shouldn't have any audible effect, unless you've managed to make O2 unstable.
Which is possible, OPA627 is faster and less stable than NE5532, it's also worse at driving capacitive loads, as typical.
OPA627 is further from rails, so you're wasting quite a bit of gain capability. In case you run out, it clips pretty horribly.
It's a part for the gain stage of an old-style analog scope and not audio.
Summary: It might sound different, but that would mean it's introducing distortion - most likely ringing or oscillation. Not to mention forcing you to pay through the nose.
Edited by AstralStorm - 7/1/12 at 11:02pm