John:
your point about the difference between aluminum and CF is well made I think.
But what interests me is your finding that you get better results hand holding, and I think your point about the damping effect of one's hands is the key here. My hands don't vibrate at a high frequency and as you note, hand tissue deadens vibrations - this does not imply however that I'm not shaky. As they kindly say, I'm getting on in years.
That's another reason for getting the Mark II version of this lens - it has Canon's so-called fourth generation IS which they claim up to four stops gain - real testing by Photozone indicates that three stops is quite safe.
Glenn