The idea for the swiftstrike line is interesting, and I would like the developers to create a complete line for the shield.
Naturally, there will be nerfs to the current shield, and it goes as follows
3* Swiftstrike Buckler Increased attack speed low, same defense
4*Agile Adarga (yes, that's an actual shield name) Increased attack speed low/medium, same normal defense as a dark thorn shield.
5*Tempest Targe Increased attack speed medium/high, as much normal defense as a Barbourous Throrn Shield.
Any questions?
I like this idea and have wanted it implemented for a while. Only thing is to keep it equal with equilavent items. Such that Wolver line and Bristling Buckler have only a Low effect until the 5* variant which has a medium bonus. So Swiftstrike and Agile = Low while Tempest = Medium and each step the armour rating increases.