I would recommend using 5.5 Update 1 due to several discussions around here referring the latest drivers for B120i/B320i included in 5.5 Update 2 or version 6 HP customized.
The latest drivers released by HP seem to have a bug which causes performance problems at least with (the big) Proliant servers Gen8.
I experienced same trouble with my B320i in a DL380e Gen8 and I'm still using 5.5 Update 1 version without any issues.
At first do update all BIOS/firmware on your Microserver prior to any OS installation.