If you are using 9 sensitivity with .55 acceleration with 5700dpi (verify in SetPoint) you are easily exceeding the maximum turn speed (you should be seeing it turn solid black). Turn Acceleration off (0) and please review the "Turning" instructional video.
Yeah, I watched that, and actually had the acceleration at 0 to start. And verified it's at 5700dpi. It takes slightly more horizontal distance for me to do a 360 when it's at 0 though, so I tried it back at the default of .55 before making the post.
I suppose to clarify that distance is what the front of the mouse travels. I can do a 180 keeping my wrist relatively in place but twisting the angle of my hand from the limits of it turned from one side to the limits of the opposite. But it does seem to require a full twist just to do 180, and so if I need to keep going beyond that I'm either lifting the mouse or moving my whole forearm at that point.
Again, this is probably just be me getting used to it and Reach's slow max turn speed (I can peg my controller's analog stick and it doesn't move any faster). Watching the video it looks like you're doing pretty much the same maximum range wrist turn and only getting 180 out of it.
What exactly does acceleration do? Based on the quick start guide of what to use it for I'm assuming it gets you closer (or at) the max thumbstick position when you move it above a certain threshold speed? Or is it more of a ramp up than an absolute threshold?
Thanks as always for your incredible responsiveness here. And the fine fine product of course.

Edit: Vehicles in Reach seem horrendous to deal with with a mouse... aiming at stuff in front isn't bad but their max turn rate seems so very low that it's a crapton of mouse pickups to say do a 180 gunning in the warthog. Switching to a secondary 100 sensitivity value doesn't really seem to help much... it seems they were just made for holding the @#$% analog stick railed against far left or right.

2nd Edit: it would be really nice if the display indicating which sensitivity you were currently using (it could simply display the value).