I have TWO xw8600s (yes, i DO like these beasts!!); one I use as an ESXi box and the other as a Windows Server 2012R2 server. Both of them use 8GB memory sticks with absolutely no issues whatsoever.
The ESXi box has the memory riser cards and a mix of 8 & 4GB memory sticks for a total of 96GB which is PLENTY for the VMs that I run. At some point I may "bite the bullet" and purchase an additional 8 sticks of 8GB RAM but that's for another posting. BEFORE I purchased the memory riser cards I had the first 8 sticks of 8GM RAM installed directly onto the Mboard. These ran for several weeks w/o any problem.
The other xw8600 box was originally one of my Windows 10 workstations that I re-purposed as a Server. For that box I purchased eight 8GB sticks of RAM that are installed directly onto the Mboard. It's been running about a week now with no issues so far.
My 2 cents.
