Sigma 120-400 users

[squid]

few questions :

- Have you noticed maybe blurring with speeds over 1/500 because of OS ? (There are stories supported even by Thom Hogan about VR causing such issues @ speeds faster than 1/500 http://www.bythom.com/nikon-vr.htm )
- Which OS mode is more suitable for (mainly horizontal) panning ? 1 or 2 ? (is maybe "1" equivalent to VR "Normal" and "2" to VR" Active"?)
- Is 400 the actual focal length @ 400mm ? (or maybe smth like 380mm ?)
- Which F is the optimum for this lens across the focal range? F8 ?


Thanks