There was quite a lengthy discussion about this a few weeks ago in the UE4 Slackers Discord - the general consensus is that passing by const value is the correct method due to the indirection cost of a reference.
It’s also worth pointing out that FName size actually changes depending on the build type you are using. In Editor they are larger, because they are case-sensitive - whereas outside of editor they are case-insensitive. In Editor they are 12 bytes, but outside of editor they are actually 8 bytes.