The impact to performance really depends on how many polys your mesh is and how many meshes you are using. Disabling backface culling on low poly meshes should impact performance that much. The best way to find out is to disable backface culling on the meshes you want it disabled and see the impact to performance yourself.
I want to disable BF culling for one mesh only, main character mesh ~4k polys. Because it's some kind of headless wizard, so other side of hat should be not culled.
Or I just could not to display the front of the wiz, so it's solution number 2. But I'd like to see those bright red eyes and hats' shadow down his shoulders/upper chest :>
Since you have the model you can also just add those back faces manually in the editor. That would avoid having too many faces in the test. But indeed, it's not more than twice the polys, i.e. less performance drop than it would be having two of the wizards Moreover, since fillrate won't be that much of a problem, the performance impact will be much less.
Moreover, since fillrate won't be that much of a problem, the performance impact will be much less.