Something is wrong with the spellpower coefficient of rank 9 Frostbrand.

Testing naked with no talents. Used a Calamities Grasp and then swapped it for a Wrathstrike with a +50 spellpower enchant, so CG=0 spellpower and WS=511 spellpower.

Frostbrand rank 9
CG = 559-560
WS = 589-590
Delta = 31 --> Spellpower coefficient of 6%

Frostbrand rank 8
CG = 493-494
WS = 543-544
Delta = 51 --> Spellpower coefficient of 10%

Frostbrand rank 7
CG = 420-421
WS = 463-464
Delta = 43 --> Spellpower coefficient of 8.4%

As the damage from Frostbrand is being increased by 20%, according to the patch notes, an easy assumption would be that there is some bug with rank 9 and that it is meant to actually be 12% and not 6%.

I did one other test just out of curiosity. Full Enhancement raiding gear with normal raiding spec. Using Wrathstrike with the +50 spellpower enchant for mainhand and a white 2.4 speed vender mace in the offhand to avoid tainting the results with a berserker enchant proc. Flametongue enchant on the mainhand and Frostbrand on the offhand. That gave me 1906 spellpower when Unleashed Rage was active.

Frostbrand rank 9 procced for 673 damage.
Frostbrand rank 8 procced for 683 damage.