I'm going to take my own advice here and take a step back, lets look at the precedents and the math. A character with inhuman strength which is incredibly likely for anyone who has claws. especially as Iago said we could think of claws as an upgrade to inhuman strength. could instead take. Off-Hand Weapon Training, Wall of Death, or Lethal Weapon. So why don't we compare these four stunts keeping the criteria you listed in mind and see what shakes loose.
I'm arbitrarily giving all of these things equal weighting.
ClawsClaws one extra physical stress inflicted, one refresh, always usable, stacks, need high concept, cant be concealed.
+1 damage 1 Stacks 1 Always usable 1 need high concept -1 cant be concealed-1 = [1]
With inhuman strength +2 damage done [3]
Lethal WeaponLethal weapon is a flat weapon rating 2 when unarmed, and your opponent is unarmored and you need to qualify for it, one refresh , only works against some targets, stacks.
+2 damage 2 Stacks1 situationaly available -1 need other stunt -1 can be concealed 1 [1]
With inhuman strength +2 damage done [3]
Lethal weapon is also considered underpowered, at least according to the post dedicated to discussing underpowered stunts.
weapon trainingA character with Inhuman strength could certainly duel wield basterd swords if he so choose which to my understanding is a weapon three. when using "weapon training" such a character is gaining 2 extra stress of damage on all weaponry attacks for one refresh, always available and stacks.
+2 damage 2 Stacks1 Always useable1 need weapon -1 can be concealed1 [5]
With inhuman strength +2 damage done [7]
Wall of DeathWhen using wall of death [and there's no restriction on how often you can use this other then having targets] your base weapon damage is multiplied by the lessor of your successes to hit or available targets. again for one refresh, always available and stacks.
+N damage N Stacks1 situationally usable-1 need weapon -1 can be concealed 1
Base [1-1]x3 =0 or [1-5]x3=12
With inhuman strength +2 damage done [1-1]x5 =0 or [1-5]x5=20
with inhuman strength and combining duel wield [1-1]x5 =0 or [1-5]x7=28/2 =14each
where x equals the number of targets available-1 times the weapon rating used.
lets assume the max number of people in range at a time will be 5 and that we are using a weapon rating 3 bastard sword. so we will solve for the minimum and maximum amounts then.
In summation it appears that lethal blows and claws in no way stand up to duel wield or wall of death no less the two being combined. all of them appear to stack with each other and while claws is always available for use it cant be concealed and being obviously inhuman isn't exactly a good thing in the dresdenverse. lethal blows is only available if your opponent isn't wearing armor, and wall of death depends on the situation. however duel wield gives a flat 2 shifts of extra damage all the time[ especially if your designed to always have weapons on you or go the extra mile and enchant swords to teleport to your hands when needed]
My proposal would be to change claws to "Your unarmed attacks are considered to be Weapon rating one." at -0 refresh
and lethal blows to not require your opponent to be unarmored. at its current -1