I bought this board about a week ago before I found this forum post.
I'm having fairly good luck so far with the latest 1405 BIOS with a 3900X and Noctua NH-U12s cooler.
I'm absolutely convinced that some negative voltage offset is desirable at least with the amount of cooling I have on this board, with this BIOS, with this chip. Currently running -0.06875. My maximum smallest fft temps with Prime95 are around 87c, with stock auto voltage they'd bump 95c. My Cinebench score also rose, and as far as I can tell I'm still boosting to 4500 to 4600mhz with single thread loads. My package power consumption reported by HWMonitor dropped to around 171 watts from 183 watts under the worst all-core loads I can come up with (small fft prime95 with AVX2 on).
Anyway, currently my best Cinebench R20 score is around 7258, AIDA64 reported memory read speed 55163MB/s.
I've hard terrible luck with the Ryzen memory calculator so I'm not using that. I have 4 16GB DIMMS of Trident Z Neo, 18-22-22-42 Micron CRJ RAM. I'm able to run them at 16-18-18-39 timings with a tRC of 58 with no issues whatsoever with a slight voltage bump to 1.370. I've run memtest86 for 8 hours and Prime95 blend for 12 hours.
Under realistic all-core loads like Handbrake my temps are in the mid to high 70's Celsius. I get near 90c under absurd synthetic loads. I've decided to upgrade to a Noctua NH-U14s which I should have tomorrow sometime in hopes of keeping worst case temps below 85c and maybe getting slightly better boost speeds.
Needless to say the 3900X is massively faster than my old 6700k 4.3GHz all-core overclock system even for most single thread workloads so thus far I'm quite happy with it. If I had to buy the board over again I'd probably spend a bit more on a better Gigabyte model.