Quite by mistake, I set up a Windows RAID 0 using one SSD each on a different Thunderbolt 2 bus controller. When I ran the Blackmagic disk speed test on the RAID, it clocked write speeds around 1000MB/sec. Pretty much surprised me because the same two drives RAIDed together on the same T2 bus clocked at 650MB/sec.

 If your PC has 2 different TB2 controllers then the results are to be expected. MP 6,1s have 3 controllers and we often get these kinds of numbers.

On the same TB2 controller reaching only 650 MB/s, this is likely caused by the SATA chipset or how it is interacting with the TB2 controller.

Thanx for confirming, Dom. What a pleasant surprise that this works.