Hello. The memory speed of each channel depends on the memory configuration. Yes, with the BIOS update as of May 15th 2009; Single Rank and Dual Rank memory modules support up to 1333 MHz of speed. See below general information. If memory modules with different speeds are installed, they will operate at the speed of the slowest installed memory module(s).
Hi, we need to upgrade memory on our R710s. Trying to find out the recommendations for the optimal  memory configuration for the 11th gen servers without dropping the memory speed. Came across  a doc dating back to 2009 telling that we can't go beyond 1 DIMM per channel for 2x RDIMMs without dropping the speed to 1066. I'm pretty sure after one of the BIOS updates the limit was changed from 1 to 2 DIMMs per channel. Can anybody confirm or share a more recent doc? Thanks in advance. 
PE R710 optimal memory configuration