I did some tests with HP5+ in the developer Donald Qualls formulated and called "Super Soup", and found that 3200 seemed to be about right. Negative scans at http://www.flickr.com/photos/ntenny/...7620654443216/, at speeds from 1600 to 12800 (yes, really).

It's basically a strong dilution of HC-110 using Dektol stock instead of water, with the addition of some ascorbic acid. Powerful stuff, but producing surprisingly normal-looking negatives for such an extreme push.