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.
From ATTO's Thunderbolt PDF on the TB2 vs TB1 speed difference:
Thunderbolt 2 is simply Thunderbolt technology with channel bonding and full DisplayPort 1.2 (4K video support). By combining the channels together over the Thunderbolt cable, Thunderbolt 2 enables (2) 20Gbps bi‐direction channels (see right side of above diagram) instead of (4) of 10Gbps channels. Although there is less overhead, making transfers more efficient, there is no overall increase in bandwidth or total data/signal rate transfer over the cable. Since there is 20Gbps of bandwidth per channel, you can stream 4K video over Thunderbolt 2 which requires 15Gbps depending on refresh rate. Furthermore, you can bypass Thunderbolt and solely send DP 1.2 over the Thunderbolt cable as it will also serve as a Display Port cable.
Finding this news out made TB2 a bit less exciting as new tech in the Mac Pro. However, any speed increase is good for business, but I think people assumed TB2 at 20Gbps is twice as fast as TB1 at 10Gbps....I know I did.