I wrote a short paper for HPEC that included some power and performance benchmarks and analysis on the HiFive Unleashed U540 SoC [0]. The SoC isn't open source as some suggest although I believe the core is based on the open source Rocket Chip Generator [1]. It seems the greatest weakness was in the slow memory interface. The details of memory controller and configuration were proprietary when I tried to find out why it wasn't performing well.
[0] http://www.ieee-hpec.org/2018/2018program/index_htm_files/15...