Then how can you explain this:
2400 MHz - Bootable and stable (advertised RAM speed)
2666 MHz - Bootable and stable
2800 MHz - Unbootable under any voltage or timing configuration
2933 MHz - Unbootable under any voltage or timing configuration
3000 MHz - Unbootable under any voltage or timing configuration
3200 MHz - Bootable, but unstable even under elevated voltages. Not crashing per se, but ASUS RealBench keeps reporting that it's files got corrupted during 10 loops of the Heavy Multitasking test.
By what logic are those 3 frequencies entirely unbootable no matter what, but 3200 MHz is almost stable. It just goes against all logic.
From what I have learned tweaking my Sabertooth X99, HWE IMC really hates 2800/3000. It is a lot easier on the IMC to work on 3200. It has something to do with 100BCLK. Since both 2800 and 3000 requires 125BCLK while 3200 and 2666 works on 100BCLK. It is definitely something to do with the HWE IMC.
If I were you I'd spend sometime reading the RVE RAM tweaking guide over ROG forum, it helped me a lot while I was still using ADATA RAM@2800. Higher VCCSA is not always good, there is a sweet spot for each and every combination of CPU and RAM. Just going up or down in 0.05V increments for VCCSA until you can find one fairly stable.
For primary timing, loosen it to 17-18-18-35 at least. Depends on the RAM IC of your kit. Samsung IC overclocks a lot better than Hynix IC.
For UEFI settings, go to External Digi power control. Set DRAM Phase Control to "Extreme", Set DRAM current capacity to 140%. For DRAM Eventual Voltage enter something like 1.35~1.37V. For the DRAM voltage in the main AITweaker tab enter 1.37~1.38V. The higher DRAM voltage help passing DRAM training during post while maintain a working voltage in Windows(ie, "eventual")
Also, adjusting CPU power phase to either Optimized or Extreme also helps stability for higher RAM speed. At least that was the case for me. Using "Standard" I can't get my DDR4-3000 to even post.
So far 3505 UEFI is the best for Sabertooth X99. Steady RAM/L3 Cache performance increase over 3402. Boot up time has decreased drastically(now takes less than 5 seconds! It used to take ~25 seconds cold booting) And now it seems I need less vcore to keep my 4.25GHz OC stable. It used to be ~1.2V. Now it is about 1.18V. Lower voltage cooler CPU. Looks like improvement to me.