Thunderbolt 2 is NOT twice the speed of Thunderbolt 1

Thunderbolt

Surprisingly, Thunderbolt 2 has the same aggregate throughput as Thunderbolt 1, according to ATTO

ATTO knows a bit about storage interfaces. They've been making disk interfaces for years and are generally regarded as being at or near the top of performers when professionals choose their storage hardware.

So it's worth reading their document that describes the differences between Thunderbolt 1 and Thunderbolt 2.

It turns out that the main difference between the two Thunderbolts is that version 2 has channel bonding and full DisplayPort 1.2, for full 4K video support. Whereas Thunderbolt 1 has 4 independent 10 Gb/s channels, Thunderbolt 2 combines these to give 2 20 Gb/s bidirectional channels. This leads to more efficient transfers, with less overhead, but the total bandwidth is unchanged.

You will need to bear this in mind when you're planning your storage and display ecosystem around your new Mac Pro.

Performance

Atto summarises the performance of Thunderbolt 2 as follows:

20 Gb/s Thunderbolt 2 technology-enabled devices are backward compatible with 10 Gb/s technology-enabled hosts.
Overall aggregate performance for Thunderbolt 2 is the same as Thunderbolt 1.
A single Thunderbolt 2 or multiple Displayport 1.1a connections will support 4K video performance.
Overall storage throughput still depends on the quantity and types of drives that are connected.
It's not practical to daisychain a 4K monitor and high-speed storage. Independent connections are required.

Thanks to Jigsaw24 for bringing this to our attention.