This can assist in determining optimal memory.
http://www.dell.com/downloads/emea/products/pedge/en/installing_upgrading_ddr3_memory_11g.pdf
As you stated with dual procs and 16 GB DIMMs you want the first slot of each channel. With dual or quad rank DIMMs there are other cravats. Such as with quad-rank DIMMs only the first two slots (A1/A2 or B1/B2) can be used due to the limitation of total ranks per channel is 8.:o(. Also as you seem aware adding more memory to other slots just drops the speed.
Also with a DELL Repository CD/DVD (SUU disc) you will need to press <F10> on boot up. In BIOS select Platform Update and walk thru the remaining options to flash the BIOS/Firmware. The current bundle set for the R710 seems to be v4.7.0 and can be downloaded using DRM and then burnt to local media.