16GB DIMM's were not originally supported, which is why they weren't included in the manual, but they are fully supported (with updated BIOS):

http://ftp.dell.com/FOLDER01656617M/1/PER710_BIOS%2006%2004%2000%20Release%20Notes.txt

http://www.dell.com/downloads/global/products/pedge/en/server-poweredge-r710-specs-en.pdf