True and usually I don’t answer questions like this with out some kind of context in which a performance hit would even be noticeable but since performance hit was part of the question well you know. 
As an accurate and safe response to the question Yes there is a performance hit but as to theory falls under the category that if I spit into a swimming pool I made the water rise. Guess it would be easier to say the 2-side materials performance is only relative to the context in which it is used. .
Humm I would say the first realization is that there is no spoon.
The next realization is that a polygon is an invention that makes mesh editing easier in your application of choice and the reality a polygon is made from two tris with a hidden edge. That means when you import your 1000 poly object into Unreal 4 it turns into 2000 surfaces as a triangle aka tris.
Does that mean 4000 tris surface count if using 2-sided ?
As an area that needs to be filled the direct answer is yes of course
Just for completeness what really matters is the simple vertex that is defied by it’s position in 3d space. Add another you can draw an edge and another you have a area that can be defined as a surface. With that bit of information you the send the small amount data to the GPU and tell it what materials to apply that has been upload to it’s vram.
Need performance improvement buy a faster video card.
Totally agree but when asked a simple question with out context how far down the rabbit hole should the response be?
Does 2sided materials add a performance hit?
Yes.
Done.
Maybe the FAQ response should be please post the question in context form?
After that I would be more than happy to talk about how little I know 