ZedBoard and MicroZed both use DDR3L because that is the memory that is most readily available from Micron. The DDR3L is fully backward compatible with DDR3 when the DDR3L is operated at 1.5V. (See Micron document TN-41-14: DDR3 SDRAM 1.35V-to-1.5V Compatibility Introduction)
At the time ZedBoard and MicroZed were designed, Xilinx did not support operating Vcco_ddr at 1.35V. In the datasheet now, you will see that Vcco_ddr has recommended operating conditions at both 1.5V and 1.35V.
After your posting, our engineering team ran a simple test to check MicroZed compatibility with Vcco_ddr = 1.35V. Changing R39 to 18.7K yields a measured value of 1.356 on the "1.5V" rail. The MicroZed passed all functional tests from QSPI and SD card.
It probably would have been more accurate to regenerate the hardware platform and FSBL after adjusting the memory parameters in the Block Design editor. Simply running the existing tests with Vcco_ddr = 1.35V provides one data point.
Again, if you are considering designing MicroZed into your product and having Vcco_ddr = 1.35V is critical, this can very likely be accommodated on your production order by making arrangements at customize@avnet.com.
