Andy - there have been several threads on this topic, including a couple of very interesting ones on how to build your own ClarityAloft-like headset. Yes, the gang here is very innovative!
To answer your question, the movement of the Halo is something I've found to be fairly minimal, but I make this statement with one major qualification. It doesn't move IF you have the cord clip attached to your shirt collar, shoulder seam or something similar. If you don't have the cord attached to your body then my experience has shown the mic position will change. I've got into the habit of clipping the cord to my left shoulder or the left side of the collar of my shirt and the operating norm for me now is to expect to never have to adjust the mic position unless I have to move it to take a drink of water or to eat something. It just stays put. After almost 30 years of being a fan of the big green mellon squasher headsets I am still amazed at the comfort and sound quality of the Halo.
Oh yeah, one other thing I should mention... The earplugs on the Halo don't last forever. Mine are now a few months old and need to be replaced. Do a search on the board here and you'll find a discussion started by Paul "Ironflight" on how to make your own replacement earplugs for peanuts. There are several fabrication techniques discussed so I'm sure that anybody who can build an airplane is highly overqualified for the task of making replacement earplugs. ;-)