I just started testing the SiFive_E51_Coreplex_FPGA_Evaluation_v1p0_r0.mcs with release date 05/04/2017 on my Arty. It is working so far, but here are a couple of minor cleanup remarks:
The .mcs file seems to have the same bug as the older E31 .mcs file, namely that the demo program loaded with the .mcs file does not display the SiFive splash on the USB serial port as indicated in the startup guide. It only lights up an LED. Subsequently loading a corrected version of the demo_gpio program displays the SiFive splash and confirms that the memory configuration was successful and the USB serial port is also working as expected.
Minor correction to the release.txt file: although meant for the E51, the file references E31, which may tend to confuse things.