Actually, it’s not recommended to use WD Green HDD in NAS devices. To have the parallel performance with NAS devices it is recommended to use WD Red HDD.
I’m running WD Green 3TB x2 in my EX2 Ultra.
Crystal Disk Mark displayed 118MB/s read and write via my gigabit network (gigabit max throughput 125MB/s).
I don’t know if mismatched drive sizes are supported/allowed with the EX2 Ultra (my old Lenovo NAS did not function with mismatched drive sizes at all).
I bought my EX2 Ultra Diskless and then I bought 2x4 TB red HDDs to work with Raid 1 and I couldn’t be happier. It works like a charm, apart from the slow sync / access on the NAS itself.