With 7 Pcombs on (sry the others are on Expansion I'm too lazy to convert for this)
Without Conc :
Hoto DMG Hammers : 3352-3379
Void : 3586-3615
Now with conc on :
Hoto : 13-13k
Void : 14-14k
Swapped MF ring to BK :
Hoto goes 14-14k
Void goes 14-15k
With BC and BK on :
Hoto 14-15k
Void 15-15k
Reminding you this is a 10% Void.
I have no other "+Magic dmg%" source than Void (like no renewed black cleft or guardians light or opalvein).
If you really wanted to test more easily, you could respec and not fully put points into conc aura, so your BH doesn't hit 5 digit damage territory.
But +Magic dmg% is multiplicative and not additive with conc. aura.
With level 35BH and level 30 conc, Void should be 6.3% stronger w/o conc, and 3% stronger w/ conc.
With level 30BH and level 25 conc, Void should be 5.% stronger w/o cocn and 1.9% stronger w/ conc.
It's very minimal because +skills is extremely important for hdin. Better to get it on slots with minimal tradeoffs like Sling/Hellwarden where you're at least getting 10/20 FCR + the magic dmg/resist vs +1 Sk (unless you're using a 2/20 helm)