Hi "1qaz 2wsx",
Todays spinning disks have a maximum IO speed of perhaps 150 MB/s, but your actual speed depends on many factors, including where on the disk you are writing or reading. RAID 6 stripes the AJA test file over all 8 disks, so we have a maximum disk bandwidth of about 8 x 150 = 1200 MB/s. Now while a RAID writes to all disks simultaneously, the actual bandwith seen is going to be less due to writing parity, filesystem directory updates (which are often long seeks) and othher filesystem overhead. Another limiting factor is the data channel bandwidth, but with a TB3 connection it's not a limiting factor here. Another factor is block size, smaller block sizes will show lower bandwidth.
What you are seeing is very close to the maximum disk bandwidth, it looks reasonable for a RAID 6. You should see faster results with RAID5, but with todays larger disks, I'd suggest keeping the RAID5 unless this bandwidth is not sufficient for your applications.