RAID-4/5 declines in write performance starting at 7 drives, even on an 8-core 3.3 GHz Mac Pro. Note that machines with fewer CPU cores and/or one Thunderbolt 2 bus (anything but Mac Pro) might do less well than shown here (TBD). It may be a good reason for professionals to stick with at least a 6-core Mac Pro.
RAID-0 striping shows a dropoff at 8 drives also. While there is one laggard in this batch of 8, this is probably not the issue. However, MPG will be retesting the 8-drive scenario with a replacement drive that is coming.
The differences between RAID-4 and RAID-5 are less than the normal variation having to do with drive behaviors, hence one cannot conclude (conclusively) that RAID-4 is faster. However, past testing has consistently shown a slight advantage for read speeds with RAID-4.