No it looks like the opposite of what you said.
Hammers display more dmg while not under conc, but same when under. But it is rounded up, so I cant know for sure.
Lemme try with full Pcombs to see if I see a real difference once on conc.
No it looks exactly like what I said if thats the case.
You would see much bigger numbers with void if the bonus magic dmg was multiplicative with conc, but if they're both 12k then it's surely just a very minor bonus equivalent to +2 conc, while losing 1 hammer skill.
In the range of 12k, it's impossible that +10% is not at least resulting in +1k, even if you lose 1 skill.