As I understand it, Intel's support of the i7-59xx CPU's for DRAM is limited to 2600 MHz or less (maybe it is 2400 MHz as max) -- WILL Intel ever have an update for the i7-59xx series of processors that support higher frequencies than 2400 or 2600 or are we stuck permanently with workarounds?
2800 is best used with the 125 strap. The 100 strap is good with DDR4-3200, while 3000, 2800 and 2666 need a lot of voltage tuning and a good CPU memory controller to get stable. This is how the platform is, it's also why OC socket was developed. Without it, getting some of the ratios over 2600 100% stable is impossible. I've got a CPU here that has an excellent memory controller - capable of speeds over DDR4-3300, but such beasts are rare.
One needs to tune System Agent voltage, Cache voltage and DRAM voltage for BOOT in most cases to help stabilize the system.