The result show that somewhere around 6 drives, daisy chaining starts to break down, even with Thunderbolt 2 as here. Hence careful choice of which devices go on which bus is key for high performance systems (and these are hard drives, not SSDs). It might mean (for example) housing four drives in two enclosures so that each enclosure can have full bandwidth, e.g. for SSDs.
Also, in spite of its two ports, the late 2013 MacBook Pro has only one Thunderbolt bus, so it maxes out quickly. As of this writing only the 2013 Mac Pro offers more than one Thunderbolt 2 bus.