From the voltage scalling I've personally seen, and what I've read of other people's results, I'd think 4.3-4.5GHz would be a viable 24/7 clock on good water for a 3849Bxxx.
As always results will vary, even between identical batches. However, the 3849Bs do seem to be quite strong. Not quite the top of the top, but well above the more recent batches. The 39xxxxxxx dies made this year don't seem to be doing so great (with the occasional exeption), and if you order from a place with high volume/turn over, your chances of getting one of these newer/worse batches is higher.
I guess my only hopes will be Tank Guys, or a lucky trip to Fry's...