After days of tweaking around, I was finally able to get it maximum RAM speed at 3866mhz. VCCIO = 1.15v, VSSA = 1.21v in BIOS, 3866mhz RAM 16-16-16-36 2T 1.425v. Anything higher the system will refuse to post and I have the latest F9b BIOS installed. The same RAM I have can run 4266mhz in my old Asrock Z370 Extreme4 board with the same VCCIO/VSSA voltage. Memory bandwidth I lost about 1gb/sec from 4266 18-18-18-38 2T 1.425v, but right now my CPU can be stable at 1.272v 5Ghz AVX=0 where as my Asrock Z370 Extreme4 board was struggling to hit 5ghz at 1.3v AVX=0. I will try increase the cache speed tonight and see if I can get it to 47x. Overall if gigabyte can fix their low RAM speed oc problems, then it's a very solid board.
9900k @ 5Ghz 1.284v, 47x cache vccio = 1.18 / vssa = 1.22/ BeQuiet Dark Rock Pro 4 CPU Cooler / Gigabyte Z390 Aorus PRO F9b BIOS / 16GB DDR4 3200C14 @ 3866 CL16-16-16-36 1T 1.425v / Asus Strix GTX1080TI @ 2063/12000 MX4 / 128GB Samsung PM961 SSD + 4TB WD Red PRO 7200RPM HDD / LG DVDRW SATA / EVGA P2 650W 80 Plus Platinum PSU / Phanteks Enthoo Pro M Case with 3x Rosewill 140mm hyperborea high airflow fan / Windows 10 Pro /