DDR3 may not be supported due to - Motherboards available.
Don't use MSI for X99 - look no further than Asus and Giga SOC.
Broadwell-E may have the edge clock to clock. But, Haswell-E always have the advantage of Higher attainable OC - at least for the time being. Might not be true for Xeon counterparts.
The 2697 recommendation is due to just the Stock clock speed.